diff --git a/devtools/client/debugger/packages/devtools-components/src/tree.js b/devtools/client/debugger/packages/devtools-components/src/tree.js
index 5f094f5699d6..71304cd74ee1 100644
--- a/devtools/client/debugger/packages/devtools-components/src/tree.js
+++ b/devtools/client/debugger/packages/devtools-components/src/tree.js
@@ -488,7 +488,7 @@ class Tree extends Component {
     super(props);
 
     this.state = {
-      seen: new Set(),
+      autoExpanded: new Set(),
     };
 
     this.treeRef = React.createRef();
@@ -536,29 +536,40 @@ class Tree extends Component {
   }
 
   _autoExpand() {
-    const { autoExpandDepth, autoExpandNodeChildrenLimit } = this.props;
-    if (!autoExpandDepth) {
+    const {
+      autoExpandDepth,
+      autoExpandNodeChildrenLimit,
+      initiallyExpanded,
+    } = this.props;
+
+    if (!autoExpandDepth && !initiallyExpanded) {
       return;
     }
 
     // Automatically expand the first autoExpandDepth levels for new items. Do
     // not use the usual DFS infrastructure because we don't want to ignore
-    // collapsed nodes.
+    // collapsed nodes. Any initially expanded items will be expanded regardless
+    // of how deep they are.
     const autoExpand = (item, currentDepth) => {
-      if (currentDepth >= autoExpandDepth || this.state.seen.has(item)) {
+      const initial = initiallyExpanded && initiallyExpanded(item);
+
+      if (!initial && currentDepth >= autoExpandDepth) {
         return;
       }
 
       const children = this.props.getChildren(item);
       if (
+        !initial &&
         autoExpandNodeChildrenLimit &&
         children.length > autoExpandNodeChildrenLimit
       ) {
         return;
       }
 
-      this.props.onExpand(item);
-      this.state.seen.add(item);
+      if (!this.state.autoExpanded.has(item)) {
+        this.props.onExpand(item);
+        this.state.autoExpanded.add(item);
+      }
 
       const length = children.length;
       for (let i = 0; i < length; i++) {
@@ -574,6 +585,14 @@ class Tree extends Component {
       }
     } else if (length != 0) {
       autoExpand(roots[0], 0);
+
+      if (initiallyExpanded) {
+        for (let i = 1; i < length; i++) {
+          if (initiallyExpanded(roots[i])) {
+            autoExpand(roots[i], 0);
+          }
+        }
+      }
     }
   }
 
diff --git a/devtools/client/debugger/packages/devtools-reps/src/object-inspector/components/ObjectInspector.js b/devtools/client/debugger/packages/devtools-reps/src/object-inspector/components/ObjectInspector.js
index 7c28a21cfcf4..e83672ae6d30 100644
--- a/devtools/client/debugger/packages/devtools-reps/src/object-inspector/components/ObjectInspector.js
+++ b/devtools/client/debugger/packages/devtools-reps/src/object-inspector/components/ObjectInspector.js
@@ -198,6 +198,7 @@ class ObjectInspector extends Component<Props> {
       nodeExpand,
       nodeCollapse,
       recordTelemetryEvent,
+      setExpanded,
       roots,
     } = this.props;
 
@@ -210,6 +211,10 @@ class ObjectInspector extends Component<Props> {
     } else {
       nodeCollapse(item);
     }
+
+    if (setExpanded) {
+      setExpanded(item, expand);
+    }
   }
 
   focusItem(item: Node) {
@@ -249,6 +254,7 @@ class ObjectInspector extends Component<Props> {
     const {
       autoExpandAll = true,
       autoExpandDepth = 1,
+      initiallyExpanded,
       focusable = true,
       disableWrap = false,
       expandedPaths,
@@ -264,6 +270,7 @@ class ObjectInspector extends Component<Props> {
 
       autoExpandAll,
       autoExpandDepth,
+      initiallyExpanded,
 
       isExpanded: item => expandedPaths && expandedPaths.has(item.path),
       isExpandable: this.isNodeExpandable,
diff --git a/devtools/client/debugger/src/actions/pause/expandScopes.js b/devtools/client/debugger/src/actions/pause/expandScopes.js
new file mode 100644
index 000000000000..034d8098582b
--- /dev/null
+++ b/devtools/client/debugger/src/actions/pause/expandScopes.js
@@ -0,0 +1,25 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at <http://mozilla.org/MPL/2.0/>. */
+
+// @flow
+
+import { getScopeItemPath } from "../../utils/pause/scopes/utils";
+import type { ThunkArgs } from "../types";
+import type { ThreadContext } from "../../types";
+
+export function setExpandedScope(
+  cx: ThreadContext,
+  item: Object,
+  expanded: boolean
+) {
+  return function({ dispatch, getState }: ThunkArgs) {
+    return dispatch({
+      type: "SET_EXPANDED_SCOPE",
+      cx,
+      thread: cx.thread,
+      path: getScopeItemPath(item),
+      expanded,
+    });
+  };
+}
diff --git a/devtools/client/debugger/src/actions/pause/index.js b/devtools/client/debugger/src/actions/pause/index.js
index f41bc22667fb..7dddedfe255a 100644
--- a/devtools/client/debugger/src/actions/pause/index.js
+++ b/devtools/client/debugger/src/actions/pause/index.js
@@ -28,3 +28,4 @@ export { pauseOnExceptions } from "./pauseOnExceptions";
 export { selectFrame } from "./selectFrame";
 export { toggleSkipPausing } from "./skipPausing";
 export { toggleMapScopes } from "./mapScopes";
+export { setExpandedScope } from "./expandScopes";
diff --git a/devtools/client/debugger/src/actions/pause/moz.build b/devtools/client/debugger/src/actions/pause/moz.build
index b25f8576b0cd..47fd0291b717 100644
--- a/devtools/client/debugger/src/actions/pause/moz.build
+++ b/devtools/client/debugger/src/actions/pause/moz.build
@@ -11,6 +11,7 @@ CompiledModules(
     'breakOnNext.js',
     'commands.js',
     'continueToHere.js',
+    'expandScopes.js',
     'fetchScopes.js',
     'index.js',
     'mapFrames.js',
diff --git a/devtools/client/debugger/src/actions/types/PauseAction.js b/devtools/client/debugger/src/actions/types/PauseAction.js
index ba6cef19831d..9e1fce19e4c8 100644
--- a/devtools/client/debugger/src/actions/types/PauseAction.js
+++ b/devtools/client/debugger/src/actions/types/PauseAction.js
@@ -147,4 +147,11 @@ export type PauseAction =
   | {|
       +type: "TOGGLE_MAP_SCOPES",
       +mapScopes: boolean,
+    |}
+  | {|
+      +type: "SET_EXPANDED_SCOPE",
+      +cx: ThreadContext,
+      +thread: string,
+      +path: string,
+      +expanded: boolean,
     |};
diff --git a/devtools/client/debugger/src/components/SecondaryPanes/Scopes.js b/devtools/client/debugger/src/components/SecondaryPanes/Scopes.js
index 83f18260a544..676509443682 100644
--- a/devtools/client/debugger/src/components/SecondaryPanes/Scopes.js
+++ b/devtools/client/debugger/src/components/SecondaryPanes/Scopes.js
@@ -13,17 +13,18 @@ import {
   getSelectedFrame,
   getGeneratedFrameScope,
   getOriginalFrameScope,
-  getIsPaused,
   getPauseReason,
   isMapScopesEnabled,
-  getCurrentThread,
+  getThreadContext,
+  getLastExpandedScopes,
 } from "../../selectors";
 import { getScopes } from "../../utils/pause/scopes";
+import { getScopeItemPath } from "../../utils/pause/scopes/utils";
 
 // eslint-disable-next-line import/named
 import { objectInspector } from "devtools-reps";
 
-import type { Why } from "../../types";
+import type { ThreadContext, Why } from "../../types";
 import type { NamedValue } from "../../utils/pause/scopes/types";
 
 import "./Scopes.css";
@@ -31,7 +32,7 @@ import "./Scopes.css";
 const { ObjectInspector } = objectInspector;
 
 type Props = {
-  isPaused: boolean,
+  cx: ThreadContext,
   selectedFrame: Object,
   generatedFrameScopes: Object,
   originalFrameScopes: Object | null,
@@ -43,6 +44,8 @@ type Props = {
   highlightDomElement: typeof actions.highlightDomElement,
   unHighlightDomElement: typeof actions.unHighlightDomElement,
   toggleMapScopes: typeof actions.toggleMapScopes,
+  setExpandedScope: typeof actions.setExpandedScope,
+  expandedScopes: string[],
 };
 
 type State = {
@@ -71,12 +74,12 @@ class Scopes extends PureComponent<Props, State> {
 
   componentWillReceiveProps(nextProps) {
     const {
-      isPaused,
+      cx,
       selectedFrame,
       originalFrameScopes,
       generatedFrameScopes,
     } = this.props;
-    const isPausedChanged = isPaused !== nextProps.isPaused;
+    const isPausedChanged = cx.isPaused !== nextProps.cx.isPaused;
     const selectedFrameChanged = selectedFrame !== nextProps.selectedFrame;
     const originalFrameScopesChanged =
       originalFrameScopes !== nextProps.originalFrameScopes;
@@ -110,19 +113,25 @@ class Scopes extends PureComponent<Props, State> {
 
   renderScopesList() {
     const {
-      isPaused,
+      cx,
       isLoading,
       openLink,
       openElementInInspector,
       highlightDomElement,
       unHighlightDomElement,
       mapScopesEnabled,
+      setExpandedScope,
+      expandedScopes,
     } = this.props;
     const { originalScopes, generatedScopes, showOriginal } = this.state;
 
     const scopes =
       (showOriginal && mapScopesEnabled && originalScopes) || generatedScopes;
 
+    function initiallyExpanded(item) {
+      return expandedScopes.some(path => path == getScopeItemPath(item));
+    }
+
     if (scopes && scopes.length > 0 && !isLoading) {
       return (
         <div className="pane scopes-list">
@@ -138,13 +147,15 @@ class Scopes extends PureComponent<Props, State> {
             onInspectIconClick={grip => openElementInInspector(grip)}
             onDOMNodeMouseOver={grip => highlightDomElement(grip)}
             onDOMNodeMouseOut={grip => unHighlightDomElement(grip)}
+            setExpanded={(path, expand) => setExpandedScope(cx, path, expand)}
+            initiallyExpanded={initiallyExpanded}
           />
         </div>
       );
     }
 
     let stateText = L10N.getStr("scopes.notPaused");
-    if (isPaused) {
+    if (cx.isPaused) {
       if (isLoading) {
         stateText = L10N.getStr("loadingText");
       } else {
@@ -165,8 +176,8 @@ class Scopes extends PureComponent<Props, State> {
 }
 
 const mapStateToProps = state => {
-  const thread = getCurrentThread(state);
-  const selectedFrame = getSelectedFrame(state, thread);
+  const cx = getThreadContext(state);
+  const selectedFrame = getSelectedFrame(state, cx.thread);
   const selectedSource = getSelectedSource(state);
 
   const {
@@ -174,7 +185,7 @@ const mapStateToProps = state => {
     pending: originalPending,
   } = getOriginalFrameScope(
     state,
-    thread,
+    cx.thread,
     selectedSource && selectedSource.id,
     selectedFrame && selectedFrame.id
   ) || { scope: null, pending: false };
@@ -184,7 +195,7 @@ const mapStateToProps = state => {
     pending: generatedPending,
   } = getGeneratedFrameScope(
     state,
-    thread,
+    cx.thread,
     selectedFrame && selectedFrame.id
   ) || {
     scope: null,
@@ -192,13 +203,14 @@ const mapStateToProps = state => {
   };
 
   return {
+    cx,
     selectedFrame,
     mapScopesEnabled: isMapScopesEnabled(state),
-    isPaused: getIsPaused(state, thread),
     isLoading: generatedPending || originalPending,
-    why: getPauseReason(state, thread),
+    why: getPauseReason(state, cx.thread),
     originalFrameScopes,
     generatedFrameScopes,
+    expandedScopes: getLastExpandedScopes(state, cx.thread),
   };
 };
 
@@ -210,5 +222,6 @@ export default connect(
     highlightDomElement: actions.highlightDomElement,
     unHighlightDomElement: actions.unHighlightDomElement,
     toggleMapScopes: actions.toggleMapScopes,
+    setExpandedScope: actions.setExpandedScope,
   }
 )(Scopes);
diff --git a/devtools/client/debugger/src/reducers/pause.js b/devtools/client/debugger/src/reducers/pause.js
index 112ef07c9430..e7a5bf5339c0 100644
--- a/devtools/client/debugger/src/reducers/pause.js
+++ b/devtools/client/debugger/src/reducers/pause.js
@@ -64,6 +64,18 @@ type ThreadPauseState = {
     },
   },
   selectedFrameId: ?string,
+
+  // Scope items that have been expanded in the current pause.
+  expandedScopes: Set<string>,
+
+  // Scope items that were expanded in the last pause. This is separate from
+  // expandedScopes so that (a) the scope pane's ObjectInspector does not depend
+  // on the current expanded scopes and we don't have to re-render the entire
+  // ObjectInspector when an element is expanded or collapsed, and (b) so that
+  // the expanded scopes are regenerated when we pause at a new location and we
+  // don't have to worry about pruning obsolete scope entries.
+  lastExpandedScopes: string[],
+
   command: Command,
   lastCommand: Command,
   wasStepping: boolean,
@@ -120,6 +132,8 @@ const createInitialPauseState = () => ({
   command: null,
   lastCommand: null,
   previousLocation: null,
+  expandedScopes: new Set(),
+  lastExpandedScopes: [],
 });
 
 function getThreadPauseState(state: PauseState, thread: ThreadId) {
@@ -292,6 +306,8 @@ function update(
       return updateThreadState({
         ...resumedPauseState,
         wasStepping: !!action.wasStepping,
+        expandedScopes: new Set(),
+        lastExpandedScopes: [...threadState().expandedScopes],
       });
     }
 
@@ -315,7 +331,7 @@ function update(
         },
         threads: {
           [action.mainThread.actor]: {
-            ...state.threads[action.mainThread.actor],
+            ...getThreadPauseState(state, action.mainThread.actor),
             ...resumedPauseState,
           },
         },
@@ -334,6 +350,17 @@ function update(
       prefs.mapScopes = mapScopes;
       return { ...state, mapScopes };
     }
+
+    case "SET_EXPANDED_SCOPE": {
+      const { path, expanded } = action;
+      const expandedScopes = new Set(threadState().expandedScopes);
+      if (expanded) {
+        expandedScopes.add(path);
+      } else {
+        expandedScopes.delete(path);
+      }
+      return updateThreadState({ expandedScopes });
+    }
   }
 
   return state;
@@ -582,4 +609,8 @@ export function getChromeScopes(state: State, thread: ThreadId) {
   return frame ? frame.scopeChain : undefined;
 }
 
+export function getLastExpandedScopes(state: State, thread: ThreadId) {
+  return getThreadPauseState(state.pause, thread).lastExpandedScopes;
+}
+
 export default update;
diff --git a/devtools/client/debugger/src/utils/pause/scopes/utils.js b/devtools/client/debugger/src/utils/pause/scopes/utils.js
index 74ad486c0cdc..d7258b4906c7 100644
--- a/devtools/client/debugger/src/utils/pause/scopes/utils.js
+++ b/devtools/client/debugger/src/utils/pause/scopes/utils.js
@@ -51,3 +51,10 @@ export function getThisVariable(this_: any, path: string): ?NamedValue {
     contents: { value: this_ },
   };
 }
+
+// Get a string path for an scope item which can be used in different pauses for
+// a thread.
+export function getScopeItemPath(item: Object) {
+  // Calling toString() on item.path allows symbols to be handled.
+  return item.path.toString();
+}
diff --git a/devtools/client/debugger/test/mochitest/browser.ini b/devtools/client/debugger/test/mochitest/browser.ini
index e2597721d0dd..4e1869f9cf22 100644
--- a/devtools/client/debugger/test/mochitest/browser.ini
+++ b/devtools/client/debugger/test/mochitest/browser.ini
@@ -150,3 +150,4 @@ skip-if = (os == 'linux' && debug) || (os == 'linux' && asan) || ccov  #Bug 1456
 [browser_dbg-scopes-xrays.js]
 [browser_dbg-merge-scopes.js]
 [browser_dbg-message-run-to-completion.js]
+[browser_dbg-remember-expanded-scopes.js]
diff --git a/devtools/client/debugger/test/mochitest/browser_dbg-inspector-integration.js b/devtools/client/debugger/test/mochitest/browser_dbg-inspector-integration.js
index 9bf564097bfb..0f5d3848886c 100644
--- a/devtools/client/debugger/test/mochitest/browser_dbg-inspector-integration.js
+++ b/devtools/client/debugger/test/mochitest/browser_dbg-inspector-integration.js
@@ -10,8 +10,8 @@ function waitForInspectorPanelChange(dbg) {
 
   return new Promise(resolve => {
     toolbox.getPanelWhenReady("inspector").then(() => {
-      ok(toolbox.inspector, "Inspector is shown.");
-      resolve(toolbox.inspector);
+      ok(toolbox.inspectorFront, "Inspector is shown.");
+      resolve(toolbox.inspectorFront);
     });
   });
 }
@@ -31,9 +31,14 @@ add_task(async function() {
 
   // Ensure hovering over button highlights the node in content pane
   const view = inspectorNode.ownerDocument.defaultView;
-  const onNodeHighlight = toolbox.target.once("inspector")
+  const onNodeHighlight = toolbox.target
+    .once("inspector")
     .then(inspector => inspector.highlighter.once("node-highlight"));
-  EventUtils.synthesizeMouseAtCenter(inspectorNode, {type: "mousemove"}, view);
+  EventUtils.synthesizeMouseAtCenter(
+    inspectorNode,
+    { type: "mousemove" },
+    view
+  );
   const nodeFront = await onNodeHighlight;
   is(nodeFront.displayName, "button", "The correct node was highlighted");
 
diff --git a/devtools/client/debugger/test/mochitest/browser_dbg-remember-expanded-scopes.js b/devtools/client/debugger/test/mochitest/browser_dbg-remember-expanded-scopes.js
new file mode 100644
index 000000000000..7e29edd8fd35
--- /dev/null
+++ b/devtools/client/debugger/test/mochitest/browser_dbg-remember-expanded-scopes.js
@@ -0,0 +1,44 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at <http://mozilla.org/MPL/2.0/>. */
+
+// Ignore strange errors when shutting down.
+const { PromiseTestUtils } = ChromeUtils.import(
+  "resource://testing-common/PromiseTestUtils.jsm"
+);
+PromiseTestUtils.whitelistRejectionsGlobally(/No such actor/);
+PromiseTestUtils.whitelistRejectionsGlobally(/connection just closed/);
+
+const MaxItems = 10;
+
+function findNode(dbg, text) {
+  for (let index = 0; index < MaxItems; index++) {
+    var elem = findElement(dbg, "scopeNode", index);
+    if (elem && elem.innerText == text) {
+      return elem;
+    }
+  }
+  return null;
+}
+
+async function toggleNode(dbg, text) {
+  const node = await waitUntilPredicate(() => findNode(dbg, text));
+  return toggleObjectInspectorNode(node);
+}
+
+// Test that expanded scopes stay expanded after resuming and pausing again.
+add_task(async function() {
+  const dbg = await initDebugger("doc-remember-expanded-scopes.html");
+  invokeInTab("main", "doc-remember-expanded-scopes.html");
+  await waitForPaused(dbg);
+
+  const MaxItems = 10;
+
+  await toggleNode(dbg, "object");
+  await toggleNode(dbg, "innerObject");
+  await stepOver(dbg);
+  await waitForPaused(dbg);
+
+  await waitUntil(() => findNode(dbg, "innerData"));
+  ok("Inner object data automatically expanded after stepping");
+});
diff --git a/devtools/client/debugger/test/mochitest/examples/doc-remember-expanded-scopes.html b/devtools/client/debugger/test/mochitest/examples/doc-remember-expanded-scopes.html
new file mode 100644
index 000000000000..7648b1841c2f
--- /dev/null
+++ b/devtools/client/debugger/test/mochitest/examples/doc-remember-expanded-scopes.html
@@ -0,0 +1,7 @@
+<script>
+function main() {
+  var object = { a:0, b:1, innerObject: { innerData: 0 } };
+  debugger;
+  return 0;
+}
+</script>
diff --git a/devtools/client/definitions.js b/devtools/client/definitions.js
index 7aefc5306b50..6671645278a7 100644
--- a/devtools/client/definitions.js
+++ b/devtools/client/definitions.js
@@ -161,7 +161,7 @@ Tools.inspector = {
 
   preventClosingOnKey: true,
   onkey: function(panel, toolbox) {
-    toolbox.inspector.nodePicker.togglePicker();
+    toolbox.inspectorFront.nodePicker.togglePicker();
   },
 
   isTargetSupported: function(target) {
@@ -617,7 +617,7 @@ function createHighlightButton(highlighterName, id) {
     isTargetSupported: target => !target.chrome,
     async onClick(event, toolbox) {
       await toolbox.initInspector();
-      const highlighter = await toolbox.inspector.getOrCreateHighlighterByType(
+      const highlighter = await toolbox.inspectorFront.getOrCreateHighlighterByType(
         highlighterName
       );
       if (highlighter.isShown()) {
diff --git a/devtools/client/framework/test/browser_keybindings_01.js b/devtools/client/framework/test/browser_keybindings_01.js
index 9b250cc2613c..868c880d4d7c 100644
--- a/devtools/client/framework/test/browser_keybindings_01.js
+++ b/devtools/client/framework/test/browser_keybindings_01.js
@@ -104,12 +104,12 @@ add_task(async function() {
   async function inspectorShouldBeOpenAndHighlighting(inspector) {
     is(toolbox.currentToolId, "inspector", "Correct tool has been loaded");
 
-    await toolbox.inspector.nodePicker.once("picker-started");
+    await toolbox.inspectorFront.nodePicker.once("picker-started");
 
     ok(true, "picker-started event received, highlighter started");
     inspector.synthesizeKey();
 
-    await toolbox.inspector.nodePicker.once("picker-stopped");
+    await toolbox.inspectorFront.nodePicker.once("picker-stopped");
     ok(true, "picker-stopped event received, highlighter stopped");
   }
 
diff --git a/devtools/client/framework/toolbox.js b/devtools/client/framework/toolbox.js
index 28f887d7f5bc..f1de4afd038c 100644
--- a/devtools/client/framework/toolbox.js
+++ b/devtools/client/framework/toolbox.js
@@ -496,7 +496,7 @@ Toolbox.prototype = {
    * Get the toolbox's inspector front. Note that it may not always have been
    * initialized first. Use `initInspector()` if needed.
    */
-  get inspector() {
+  get inspectorFront() {
     return this._inspector;
   },
 
@@ -1720,10 +1720,10 @@ Toolbox.prototype = {
     if (currentPanel.togglePicker) {
       currentPanel.togglePicker(focus);
     } else {
-      if (!this.inspector) {
+      if (!this.inspectorFront) {
         await this.initInspector();
       }
-      this.inspector.nodePicker.togglePicker(focus);
+      this.inspectorFront.nodePicker.togglePicker(focus);
     }
   },
 
@@ -1737,7 +1737,7 @@ Toolbox.prototype = {
       if (currentPanel.cancelPicker) {
         currentPanel.cancelPicker();
       } else {
-        this.inspector.nodePicker.cancel();
+        this.inspectorFront.nodePicker.cancel();
       }
       // Stop the console from toggling.
       event.stopImmediatePropagation();
@@ -1748,7 +1748,7 @@ Toolbox.prototype = {
     this.tellRDMAboutPickerState(true);
     this.pickerButton.isChecked = true;
     await this.selectTool("inspector", "inspect_dom");
-    this.on("select", this.inspector.nodePicker.stop);
+    this.on("select", this.inspectorFront.nodePicker.stop);
   },
 
   _onPickerStarted: async function() {
@@ -1758,7 +1758,7 @@ Toolbox.prototype = {
 
   _onPickerStopped: function() {
     this.tellRDMAboutPickerState(false);
-    this.off("select", this.inspector.nodePicker.stop);
+    this.off("select", this.inspectorFront.nodePicker.stop);
     this.doc.removeEventListener("keypress", this._onPickerKeypress, true);
     this.pickerButton.isChecked = false;
   },
@@ -1881,7 +1881,7 @@ Toolbox.prototype = {
    * Update the buttons.
    */
   updateToolboxButtons() {
-    const inspector = this.inspector;
+    const inspector = this.inspectorFront;
     // two of the buttons have highlighters that need to be cleared
     // on will-navigate, otherwise we hold on to the stale highlighter
     const hasHighlighters =
@@ -3267,14 +3267,23 @@ Toolbox.prototype = {
         // TODO: replace with getFront once inspector is separated from the toolbox
         // TODO: remove these bindings
         this._inspector = await this.target.getInspector();
-        this._walker = this.inspector.walker;
-        this._highlighter = this.inspector.highlighter;
-        this._selection = this.inspector.selection;
+        this._walker = this.inspectorFront.walker;
+        this._highlighter = this.inspectorFront.highlighter;
+        this._selection = this.inspectorFront.selection;
 
-        this.inspector.nodePicker.on("picker-starting", this._onPickerStarting);
-        this.inspector.nodePicker.on("picker-started", this._onPickerStarted);
-        this.inspector.nodePicker.on("picker-stopped", this._onPickerStopped);
-        this.inspector.nodePicker.on(
+        this.inspectorFront.nodePicker.on(
+          "picker-starting",
+          this._onPickerStarting
+        );
+        this.inspectorFront.nodePicker.on(
+          "picker-started",
+          this._onPickerStarted
+        );
+        this.inspectorFront.nodePicker.on(
+          "picker-stopped",
+          this._onPickerStopped
+        );
+        this.inspectorFront.nodePicker.on(
           "picker-node-canceled",
           this._onPickerCanceled
         );
diff --git a/devtools/client/inspector/animation/animation.js b/devtools/client/inspector/animation/animation.js
index d7612f34f508..3388e4658b37 100644
--- a/devtools/client/inspector/animation/animation.js
+++ b/devtools/client/inspector/animation/animation.js
@@ -150,11 +150,11 @@ class AnimationInspector {
     this.provider = provider;
 
     this.inspector.sidebar.on("select", this.onSidebarSelectionChanged);
-    this.inspector.inspector.nodePicker.on(
+    this.inspector.inspectorFront.nodePicker.on(
       "picker-started",
       this.onElementPickerStarted
     );
-    this.inspector.inspector.nodePicker.on(
+    this.inspector.inspectorFront.nodePicker.on(
       "picker-stopped",
       this.onElementPickerStopped
     );
@@ -183,11 +183,11 @@ class AnimationInspector {
       "inspector-sidebar-resized",
       this.onSidebarResized
     );
-    this.inspector.inspector.nodePicker.off(
+    this.inspector.inspectorFront.nodePicker.off(
       "picker-started",
       this.onElementPickerStarted
     );
-    this.inspector.inspector.nodePicker.off(
+    this.inspector.inspectorFront.nodePicker.off(
       "picker-stopped",
       this.onElementPickerStopped
     );
@@ -711,7 +711,7 @@ class AnimationInspector {
   }
 
   toggleElementPicker() {
-    this.inspector.inspector.nodePicker.togglePicker();
+    this.inspector.inspectorFront.nodePicker.togglePicker();
   }
 
   async update() {
diff --git a/devtools/client/inspector/fonts/fonts.js b/devtools/client/inspector/fonts/fonts.js
index 092caa1ac06f..f070725e01b9 100644
--- a/devtools/client/inspector/fonts/fonts.js
+++ b/devtools/client/inspector/fonts/fonts.js
@@ -852,7 +852,7 @@ class FontInspector {
   async onToggleFontHighlight(font, show, isForCurrentElement = true) {
     if (!this.fontsHighlighter) {
       try {
-        this.fontsHighlighter = await this.inspector.inspector.getHighlighterByType(
+        this.fontsHighlighter = await this.inspector.inspectorFront.getHighlighterByType(
           "FontsHighlighter"
         );
       } catch (e) {
diff --git a/devtools/client/inspector/inspector.js b/devtools/client/inspector/inspector.js
index 794b6ff8c792..0ebb93f020d0 100644
--- a/devtools/client/inspector/inspector.js
+++ b/devtools/client/inspector/inspector.js
@@ -227,8 +227,8 @@ Inspector.prototype = {
     return this._toolbox;
   },
 
-  get inspector() {
-    return this.toolbox.inspector;
+  get inspectorFront() {
+    return this.toolbox.inspectorFront;
   },
 
   get walker() {
@@ -417,7 +417,7 @@ Inspector.prototype = {
   },
 
   _getPageStyle: function() {
-    return this.inspector.getPageStyle().then(pageStyle => {
+    return this.inspectorFront.getPageStyle().then(pageStyle => {
       this.pageStyle = pageStyle;
     }, this._handleRejectionIfNotDestroyed);
   },
@@ -1208,7 +1208,7 @@ Inspector.prototype = {
    */
   async supportsEyeDropper() {
     try {
-      return await this.inspector.supportsHighlighters();
+      return await this.inspectorFront.supportsHighlighters();
     } catch (e) {
       console.error(e);
       return false;
@@ -1706,14 +1706,14 @@ Inspector.prototype = {
   },
 
   startEyeDropperListeners: function() {
-    this.inspector.once("color-pick-canceled", this.onEyeDropperDone);
-    this.inspector.once("color-picked", this.onEyeDropperDone);
+    this.inspectorFront.once("color-pick-canceled", this.onEyeDropperDone);
+    this.inspectorFront.once("color-picked", this.onEyeDropperDone);
     this.walker.once("new-root", this.onEyeDropperDone);
   },
 
   stopEyeDropperListeners: function() {
-    this.inspector.off("color-pick-canceled", this.onEyeDropperDone);
-    this.inspector.off("color-picked", this.onEyeDropperDone);
+    this.inspectorFront.off("color-pick-canceled", this.onEyeDropperDone);
+    this.inspectorFront.off("color-picked", this.onEyeDropperDone);
     this.walker.off("new-root", this.onEyeDropperDone);
   },
 
@@ -1736,7 +1736,7 @@ Inspector.prototype = {
     this.telemetry.scalarSet(TELEMETRY_EYEDROPPER_OPENED, 1);
     this.eyeDropperButton.classList.add("checked");
     this.startEyeDropperListeners();
-    return this.inspector
+    return this.inspectorFront
       .pickColorFromPage({ copyOnSelect: true })
       .catch(console.error);
   },
@@ -1753,7 +1753,7 @@ Inspector.prototype = {
 
     this.eyeDropperButton.classList.remove("checked");
     this.stopEyeDropperListeners();
-    return this.inspector.cancelPickColorFromPage().catch(console.error);
+    return this.inspectorFront.cancelPickColorFromPage().catch(console.error);
   },
 
   /**
diff --git a/devtools/client/inspector/markup/markup-context-menu.js b/devtools/client/inspector/markup/markup-context-menu.js
index e947d1eef807..28cb6b436e6c 100644
--- a/devtools/client/inspector/markup/markup-context-menu.js
+++ b/devtools/client/inspector/markup/markup-context-menu.js
@@ -79,7 +79,7 @@ class MarkupContextMenu {
    * This method is here for the benefit of copying links.
    */
   _copyAttributeLink(link) {
-    this.inspector.inspector
+    this.inspector.inspectorFront
       .resolveRelativeURL(link, this.selection.nodeFront)
       .then(url => {
         clipboardHelper.copyString(url);
diff --git a/devtools/client/inspector/markup/markup.js b/devtools/client/inspector/markup/markup.js
index fbc7b2b4037c..c0314c2fdb7b 100644
--- a/devtools/client/inspector/markup/markup.js
+++ b/devtools/client/inspector/markup/markup.js
@@ -183,11 +183,11 @@ function MarkupView(inspector, frame, controllerWindow) {
   this.walker.on("mutations", this._mutationObserver);
   this.win.addEventListener("copy", this._onCopy);
   this.win.addEventListener("mouseup", this._onMouseUp);
-  this.inspector.inspector.nodePicker.on(
+  this.inspector.inspectorFront.nodePicker.on(
     "picker-node-canceled",
     this._onToolboxPickerCanceled
   );
-  this.inspector.inspector.nodePicker.on(
+  this.inspector.inspectorFront.nodePicker.on(
     "picker-node-hovered",
     this._onToolboxPickerHover
   );
@@ -932,7 +932,7 @@ MarkupView.prototype = {
 
     if (type === "uri" || type === "cssresource" || type === "jsresource") {
       // Open link in a new tab.
-      this.inspector.inspector
+      this.inspector.inspectorFront
         .resolveRelativeURL(link, this.inspector.selection.nodeFront)
         .then(url => {
           if (type === "uri") {
@@ -2245,7 +2245,7 @@ MarkupView.prototype = {
     this._elt.removeEventListener("mouseout", this._onMouseOut);
     this._frame.removeEventListener("focus", this._onFocus);
     this.inspector.selection.off("new-node-front", this._onNewSelection);
-    this.inspector.inspector.nodePicker.off(
+    this.inspector.inspectorFront.nodePicker.off(
       "picker-node-hovered",
       this._onToolboxPickerHover
     );
diff --git a/devtools/client/inspector/markup/test/browser_markup_links_04.js b/devtools/client/inspector/markup/test/browser_markup_links_04.js
index 013c2092196b..0c8fd2908b6c 100644
--- a/devtools/client/inspector/markup/test/browser_markup_links_04.js
+++ b/devtools/client/inspector/markup/test/browser_markup_links_04.js
@@ -140,7 +140,7 @@ add_task(async function() {
       info("Get link from node attribute");
       const link = await nodeFront.getAttribute(test.attributeName);
       info("Resolve link to absolue URL");
-      const expected = await inspector.inspector.resolveRelativeURL(
+      const expected = await inspector.inspectorFront.resolveRelativeURL(
         link,
         nodeFront
       );
diff --git a/devtools/client/inspector/rules/new-rules.js b/devtools/client/inspector/rules/new-rules.js
index 7125d74bd017..87ac8eef1455 100644
--- a/devtools/client/inspector/rules/new-rules.js
+++ b/devtools/client/inspector/rules/new-rules.js
@@ -328,7 +328,7 @@ class RulesView {
     }
 
     try {
-      const front = this.inspector.inspector;
+      const front = this.inspector.inspectorFront;
       this._selectorHighlighter = await front.getHighlighterByType(
         "SelectorHighlighter"
       );
diff --git a/devtools/client/inspector/rules/rules.js b/devtools/client/inspector/rules/rules.js
index de73bb1bfaf3..c9d77ee45138 100644
--- a/devtools/client/inspector/rules/rules.js
+++ b/devtools/client/inspector/rules/rules.js
@@ -322,7 +322,7 @@ CssRuleView.prototype = {
     }
 
     try {
-      const front = this.inspector.inspector;
+      const front = this.inspector.inspectorFront;
       const h = await front.getHighlighterByType("SelectorHighlighter");
       this.selectorHighlighter = h;
       return h;
diff --git a/devtools/client/inspector/rules/test/browser_rules_colorpicker-hides-element-picker.js b/devtools/client/inspector/rules/test/browser_rules_colorpicker-hides-element-picker.js
index c876b1afdd2a..be47ad1d024f 100644
--- a/devtools/client/inspector/rules/test/browser_rules_colorpicker-hides-element-picker.js
+++ b/devtools/client/inspector/rules/test/browser_rules_colorpicker-hides-element-picker.js
@@ -12,7 +12,9 @@ add_task(async function() {
   await addTab("data:text/html;charset=utf-8," + encodeURIComponent(TEST_URI));
 
   const { view, toolbox } = await openRuleView();
-  const pickerStopped = toolbox.inspector.nodePicker.once("picker-stopped");
+  const pickerStopped = toolbox.inspectorFront.nodePicker.once(
+    "picker-stopped"
+  );
 
   await startPicker(toolbox);
 
diff --git a/devtools/client/inspector/rules/test/browser_rules_eyedropper.js b/devtools/client/inspector/rules/test/browser_rules_eyedropper.js
index 6f09479587ee..fbcfe811d2d6 100644
--- a/devtools/client/inspector/rules/test/browser_rules_eyedropper.js
+++ b/devtools/client/inspector/rules/test/browser_rules_eyedropper.js
@@ -95,7 +95,7 @@ async function runTest(testActor, inspector, view) {
 async function testESC(swatch, inspector, testActor) {
   info("Press escape");
   const onCanceled = new Promise(resolve => {
-    inspector.inspector.once("color-pick-canceled", resolve);
+    inspector.inspectorFront.once("color-pick-canceled", resolve);
   });
   await testActor.synthesizeKey({ key: "VK_ESCAPE", options: {} });
   await onCanceled;
@@ -107,7 +107,7 @@ async function testESC(swatch, inspector, testActor) {
 async function testSelect(view, swatch, inspector, testActor) {
   info("Click at x:10px y:10px");
   const onPicked = new Promise(resolve => {
-    inspector.inspector.once("color-picked", resolve);
+    inspector.inspectorFront.once("color-picked", resolve);
   });
   // The change to the content is done async after rule view change
   const onRuleViewChanged = view.once("ruleview-changed");
diff --git a/devtools/client/inspector/shared/highlighters-overlay.js b/devtools/client/inspector/shared/highlighters-overlay.js
index 02890e9e7740..02fabd539078 100644
--- a/devtools/client/inspector/shared/highlighters-overlay.js
+++ b/devtools/client/inspector/shared/highlighters-overlay.js
@@ -34,7 +34,7 @@ class HighlightersOverlay {
    */
   constructor(inspector) {
     this.inspector = inspector;
-    this.inspectorFront = this.inspector.inspector;
+    this.inspectorFront = this.inspector.inspectorFront;
     this.store = this.inspector.store;
     this.target = this.inspector.target;
     this.telemetry = this.inspector.telemetry;
diff --git a/devtools/client/inspector/shared/style-inspector-menu.js b/devtools/client/inspector/shared/style-inspector-menu.js
index e89be2f00a9d..e4588ddf0c4a 100644
--- a/devtools/client/inspector/shared/style-inspector-menu.js
+++ b/devtools/client/inspector/shared/style-inspector-menu.js
@@ -414,7 +414,7 @@ StyleInspectorMenu.prototype = {
 
     let message;
     try {
-      const inspectorFront = this.inspector.inspector;
+      const inspectorFront = this.inspector.inspectorFront;
       const imageUrl = this._clickedNodeInfo.value.url;
       const data = await inspectorFront.getImageDataFromURL(imageUrl);
       message = await data.data.string();
diff --git a/devtools/client/inspector/shared/tooltips-overlay.js b/devtools/client/inspector/shared/tooltips-overlay.js
index a05028e2552b..4a24f470c452 100644
--- a/devtools/client/inspector/shared/tooltips-overlay.js
+++ b/devtools/client/inspector/shared/tooltips-overlay.js
@@ -422,7 +422,7 @@ TooltipsOverlay.prototype = {
       naturalWidth = size.naturalWidth;
       naturalHeight = size.naturalHeight;
     } else {
-      const inspectorFront = this.view.inspector.inspector;
+      const inspectorFront = this.view.inspector.inspectorFront;
       const { data, size } = await inspectorFront.getImageDataFromURL(
         imageUrl,
         maxDim
diff --git a/devtools/client/inspector/test/browser_inspector_highlighter-03.js b/devtools/client/inspector/test/browser_inspector_highlighter-03.js
index 13f9741deb12..9e1d0040509c 100644
--- a/devtools/client/inspector/test/browser_inspector_highlighter-03.js
+++ b/devtools/client/inspector/test/browser_inspector_highlighter-03.js
@@ -66,7 +66,7 @@ add_task(async function() {
   await testActor.isNodeCorrectlyHighlighted(iframeBodySelector, is);
 
   info("Waiting for the element picker to deactivate.");
-  await inspector.inspector.nodePicker.stop();
+  await inspector.inspectorFront.nodePicker.stop();
 
   function moveMouseOver(selector, x, y) {
     info("Waiting for element " + selector + " to be highlighted");
@@ -76,6 +76,6 @@ add_task(async function() {
       y,
       options: { type: "mousemove" },
     });
-    return inspector.inspector.nodePicker.once("picker-node-hovered");
+    return inspector.inspectorFront.nodePicker.once("picker-node-hovered");
   }
 });
diff --git a/devtools/client/inspector/test/browser_inspector_highlighter-07.js b/devtools/client/inspector/test/browser_inspector_highlighter-07.js
index bf8eda067687..ec680b194d88 100644
--- a/devtools/client/inspector/test/browser_inspector_highlighter-07.js
+++ b/devtools/client/inspector/test/browser_inspector_highlighter-07.js
@@ -95,6 +95,6 @@ add_task(async function() {
       y,
       options: { type: "mousemove" },
     });
-    return inspector.inspector.nodePicker.once("picker-node-hovered");
+    return inspector.inspectorFront.nodePicker.once("picker-node-hovered");
   }
 });
diff --git a/devtools/client/inspector/test/browser_inspector_highlighter-by-type.js b/devtools/client/inspector/test/browser_inspector_highlighter-by-type.js
index 1876111b0193..27d88eeca90f 100644
--- a/devtools/client/inspector/test/browser_inspector_highlighter-by-type.js
+++ b/devtools/client/inspector/test/browser_inspector_highlighter-by-type.js
@@ -18,23 +18,27 @@ add_task(async function() {
   await unknownHighlighterTypeShouldntBeAccepted(inspector);
 });
 
-async function onlyOneInstanceOfMainHighlighter({ inspector }) {
+async function onlyOneInstanceOfMainHighlighter({ inspectorFront }) {
   info("Check that the inspector always sends back the same main highlighter");
 
-  const h1 = await inspector.getHighlighter(false);
-  const h2 = await inspector.getHighlighter(false);
+  const h1 = await inspectorFront.getHighlighter(false);
+  const h2 = await inspectorFront.getHighlighter(false);
   is(h1, h2, "The same highlighter front was returned");
 
   is(h1.typeName, "highlighter", "The right front type was returned");
 }
 
-async function manyInstancesOfCustomHighlighters({ inspector }) {
-  const h1 = await inspector.getHighlighterByType("BoxModelHighlighter");
-  const h2 = await inspector.getHighlighterByType("BoxModelHighlighter");
+async function manyInstancesOfCustomHighlighters({ inspectorFront }) {
+  const h1 = await inspectorFront.getHighlighterByType("BoxModelHighlighter");
+  const h2 = await inspectorFront.getHighlighterByType("BoxModelHighlighter");
   ok(h1 !== h2, "getHighlighterByType returns new instances every time (1)");
 
-  const h3 = await inspector.getHighlighterByType("CssTransformHighlighter");
-  const h4 = await inspector.getHighlighterByType("CssTransformHighlighter");
+  const h3 = await inspectorFront.getHighlighterByType(
+    "CssTransformHighlighter"
+  );
+  const h4 = await inspectorFront.getHighlighterByType(
+    "CssTransformHighlighter"
+  );
   ok(h3 !== h4, "getHighlighterByType returns new instances every time (2)");
   ok(
     h3 !== h1 && h3 !== h2,
@@ -51,9 +55,11 @@ async function manyInstancesOfCustomHighlighters({ inspector }) {
   await h4.finalize();
 }
 
-async function showHideMethodsAreAvailable({ inspector }) {
-  const h1 = await inspector.getHighlighterByType("BoxModelHighlighter");
-  const h2 = await inspector.getHighlighterByType("CssTransformHighlighter");
+async function showHideMethodsAreAvailable({ inspectorFront }) {
+  const h1 = await inspectorFront.getHighlighterByType("BoxModelHighlighter");
+  const h2 = await inspectorFront.getHighlighterByType(
+    "CssTransformHighlighter"
+  );
 
   ok("show" in h1, "Show method is present on the front API");
   ok("show" in h2, "Show method is present on the front API");
@@ -64,7 +70,7 @@ async function showHideMethodsAreAvailable({ inspector }) {
   await h2.finalize();
 }
 
-async function unknownHighlighterTypeShouldntBeAccepted({ inspector }) {
-  const h = await inspector.getHighlighterByType("whatever");
+async function unknownHighlighterTypeShouldntBeAccepted({ inspectorFront }) {
+  const h = await inspectorFront.getHighlighterByType("whatever");
   ok(!h, "No highlighter was returned for the invalid type");
 }
diff --git a/devtools/client/inspector/test/browser_inspector_highlighter-cancel.js b/devtools/client/inspector/test/browser_inspector_highlighter-cancel.js
index a366fd8f89bc..5073393d7bbf 100644
--- a/devtools/client/inspector/test/browser_inspector_highlighter-cancel.js
+++ b/devtools/client/inspector/test/browser_inspector_highlighter-cancel.js
@@ -39,7 +39,7 @@ add_task(async function() {
   function cancelPickerByShortcut() {
     info("Key pressed. Waiting for picker to be canceled.");
     testActor.synthesizeKey({ key: "VK_ESCAPE", options: {} });
-    return inspector.inspector.nodePicker.once("picker-node-canceled");
+    return inspector.inspectorFront.nodePicker.once("picker-node-canceled");
   }
 
   function moveMouseOver(selector) {
diff --git a/devtools/client/inspector/test/browser_inspector_highlighter-cssgrid_01.js b/devtools/client/inspector/test/browser_inspector_highlighter-cssgrid_01.js
index 6300cdbbcc2b..4a670dbd6716 100644
--- a/devtools/client/inspector/test/browser_inspector_highlighter-cssgrid_01.js
+++ b/devtools/client/inspector/test/browser_inspector_highlighter-cssgrid_01.js
@@ -42,7 +42,7 @@ add_task(async function() {
   const { inspector, testActor } = await openInspectorForURL(
     "data:text/html;charset=utf-8," + encodeURIComponent(TEST_URL)
   );
-  const front = inspector.inspector;
+  const front = inspector.inspectorFront;
   const highlighter = await front.getHighlighterByType(HIGHLIGHTER_TYPE);
 
   await isHiddenByDefault(testActor, highlighter);
diff --git a/devtools/client/inspector/test/browser_inspector_highlighter-cssgrid_02.js b/devtools/client/inspector/test/browser_inspector_highlighter-cssgrid_02.js
index 9e2f1c14a509..80cb45f4e7e6 100644
--- a/devtools/client/inspector/test/browser_inspector_highlighter-cssgrid_02.js
+++ b/devtools/client/inspector/test/browser_inspector_highlighter-cssgrid_02.js
@@ -23,7 +23,7 @@ add_task(async function() {
   const { inspector, testActor } = await openInspectorForURL(
     "data:text/html;charset=utf-8," + encodeURIComponent(TEST_URL)
   );
-  const front = inspector.inspector;
+  const front = inspector.inspectorFront;
   const highlighter = await front.getHighlighterByType(HIGHLIGHTER_TYPE);
 
   info("Try to show the highlighter on the grid container");
diff --git a/devtools/client/inspector/test/browser_inspector_highlighter-cssshape_01.js b/devtools/client/inspector/test/browser_inspector_highlighter-cssshape_01.js
index d9f32f10f0ed..585858c3a2ed 100644
--- a/devtools/client/inspector/test/browser_inspector_highlighter-cssshape_01.js
+++ b/devtools/client/inspector/test/browser_inspector_highlighter-cssshape_01.js
@@ -30,7 +30,7 @@ const SHAPE_TYPES = [
 
 add_task(async function() {
   const { inspector, testActor } = await openInspectorForURL(TEST_URL);
-  const front = inspector.inspector;
+  const front = inspector.inspectorFront;
   const highlighter = await front.getHighlighterByType(HIGHLIGHTER_TYPE);
 
   await isHiddenByDefault(testActor, highlighter);
diff --git a/devtools/client/inspector/test/browser_inspector_highlighter-cssshape_02.js b/devtools/client/inspector/test/browser_inspector_highlighter-cssshape_02.js
index 1f6e0c004de5..c057229180a1 100644
--- a/devtools/client/inspector/test/browser_inspector_highlighter-cssshape_02.js
+++ b/devtools/client/inspector/test/browser_inspector_highlighter-cssshape_02.js
@@ -11,7 +11,7 @@ const HIGHLIGHTER_TYPE = "ShapesHighlighter";
 
 add_task(async function() {
   const { inspector, testActor } = await openInspectorForURL(TEST_URL);
-  const front = inspector.inspector;
+  const front = inspector.inspectorFront;
   const highlighter = await front.getHighlighterByType(HIGHLIGHTER_TYPE);
 
   await polygonHasCorrectAttrs(testActor, inspector, highlighter);
diff --git a/devtools/client/inspector/test/browser_inspector_highlighter-csstransform_01.js b/devtools/client/inspector/test/browser_inspector_highlighter-csstransform_01.js
index 12bddaafa864..088daa78de5d 100644
--- a/devtools/client/inspector/test/browser_inspector_highlighter-csstransform_01.js
+++ b/devtools/client/inspector/test/browser_inspector_highlighter-csstransform_01.js
@@ -23,7 +23,7 @@ add_task(async function() {
   const { inspector, testActor } = await openInspectorForURL(
     "data:text/html;charset=utf-8," + encodeURI(TEST_URL)
   );
-  const front = inspector.inspector;
+  const front = inspector.inspectorFront;
 
   const highlighter = await front.getHighlighterByType(
     "CssTransformHighlighter"
diff --git a/devtools/client/inspector/test/browser_inspector_highlighter-csstransform_02.js b/devtools/client/inspector/test/browser_inspector_highlighter-csstransform_02.js
index a9219eac2bac..4333d3669f50 100644
--- a/devtools/client/inspector/test/browser_inspector_highlighter-csstransform_02.js
+++ b/devtools/client/inspector/test/browser_inspector_highlighter-csstransform_02.js
@@ -22,7 +22,7 @@ const TEST_URL = URL_ROOT + "doc_inspector_highlighter_csstransform.html";
 
 add_task(async function() {
   const { inspector, testActor } = await openInspectorForURL(TEST_URL);
-  const front = inspector.inspector;
+  const front = inspector.inspectorFront;
 
   const highlighter = await front.getHighlighterByType(
     "CssTransformHighlighter"
diff --git a/devtools/client/inspector/test/browser_inspector_highlighter-iframes_01.js b/devtools/client/inspector/test/browser_inspector_highlighter-iframes_01.js
index b0c06cefaebf..931a3fd11263 100644
--- a/devtools/client/inspector/test/browser_inspector_highlighter-iframes_01.js
+++ b/devtools/client/inspector/test/browser_inspector_highlighter-iframes_01.js
@@ -70,7 +70,7 @@ add_task(async function() {
   );
 
   info("Waiting for element picker to deactivate.");
-  await inspector.inspector.nodePicker.stop();
+  await inspector.inspectorFront.nodePicker.stop();
 
   function moveMouseOver(selector) {
     info("Waiting for element " + selector + " to be highlighted");
@@ -80,6 +80,8 @@ add_task(async function() {
         options: { type: "mousemove" },
         center: true,
       })
-      .then(() => inspector.inspector.nodePicker.once("picker-node-hovered"));
+      .then(() =>
+        inspector.inspectorFront.nodePicker.once("picker-node-hovered")
+      );
   }
 });
diff --git a/devtools/client/inspector/test/browser_inspector_highlighter-keybinding_01.js b/devtools/client/inspector/test/browser_inspector_highlighter-keybinding_01.js
index f648f5aae090..ca8d5722b882 100644
--- a/devtools/client/inspector/test/browser_inspector_highlighter-keybinding_01.js
+++ b/devtools/client/inspector/test/browser_inspector_highlighter-keybinding_01.js
@@ -54,12 +54,12 @@ add_task(async function() {
   info("First child selection test Passed.");
 
   info("Stopping the picker");
-  await toolbox.inspector.nodePicker.stop();
+  await toolbox.inspectorFront.nodePicker.stop();
 
   function doKeyHover(args) {
     info("Key pressed. Waiting for element to be highlighted/hovered");
     testActor.synthesizeKey(args);
-    return toolbox.inspector.nodePicker.once("picker-node-hovered");
+    return toolbox.inspectorFront.nodePicker.once("picker-node-hovered");
   }
 
   function moveMouseOver(selector) {
@@ -69,6 +69,6 @@ add_task(async function() {
       center: true,
       selector: selector,
     });
-    return toolbox.inspector.nodePicker.once("picker-node-hovered");
+    return toolbox.inspectorFront.nodePicker.once("picker-node-hovered");
   }
 });
diff --git a/devtools/client/inspector/test/browser_inspector_highlighter-keybinding_02.js b/devtools/client/inspector/test/browser_inspector_highlighter-keybinding_02.js
index 8f684be541d5..537009116a55 100644
--- a/devtools/client/inspector/test/browser_inspector_highlighter-keybinding_02.js
+++ b/devtools/client/inspector/test/browser_inspector_highlighter-keybinding_02.js
@@ -48,12 +48,12 @@ add_task(async function() {
   info("Previously chosen child is remembered. Passed.");
 
   info("Stopping the picker");
-  await toolbox.inspector.nodePicker.stop();
+  await toolbox.inspectorFront.nodePicker.stop();
 
   function doKeyHover(args) {
     info("Key pressed. Waiting for element to be highlighted/hovered");
     const onHighlighterReady = toolbox.once("highlighter-ready");
-    const onPickerNodeHovered = toolbox.inspector.nodePicker.once(
+    const onPickerNodeHovered = toolbox.inspectorFront.nodePicker.once(
       "picker-node-hovered"
     );
     testActor.synthesizeKey(args);
@@ -63,7 +63,7 @@ add_task(async function() {
   function moveMouseOver(selector) {
     info("Waiting for element " + selector + " to be highlighted");
     const onHighlighterReady = toolbox.once("highlighter-ready");
-    const onPickerNodeHovered = toolbox.inspector.nodePicker.once(
+    const onPickerNodeHovered = toolbox.inspectorFront.nodePicker.once(
       "picker-node-hovered"
     );
     testActor.synthesizeMouse({
diff --git a/devtools/client/inspector/test/browser_inspector_highlighter-keybinding_03.js b/devtools/client/inspector/test/browser_inspector_highlighter-keybinding_03.js
index 76e54ad68e54..262cbf56bf34 100644
--- a/devtools/client/inspector/test/browser_inspector_highlighter-keybinding_03.js
+++ b/devtools/client/inspector/test/browser_inspector_highlighter-keybinding_03.js
@@ -57,20 +57,20 @@ add_task(async function() {
     return promise.all([
       inspector.selection.once("new-node-front"),
       inspector.once("inspector-updated"),
-      inspector.inspector.nodePicker.once("picker-stopped"),
+      inspector.inspectorFront.nodePicker.once("picker-stopped"),
     ]);
   }
 
   function doKeyStop(args) {
     info("Key pressed. Waiting for picker to be canceled");
     testActor.synthesizeKey(args);
-    return inspector.inspector.nodePicker.once("picker-stopped");
+    return inspector.inspectorFront.nodePicker.once("picker-stopped");
   }
 
   function moveMouseOver(selector) {
     info("Waiting for element " + selector + " to be highlighted");
     const onHighlighterReady = toolbox.once("highlighter-ready");
-    const onPickerNodeHovered = inspector.inspector.nodePicker.once(
+    const onPickerNodeHovered = inspector.inspectorFront.nodePicker.once(
       "picker-node-hovered"
     );
     testActor.synthesizeMouse({
diff --git a/devtools/client/inspector/test/browser_inspector_highlighter-keybinding_04.js b/devtools/client/inspector/test/browser_inspector_highlighter-keybinding_04.js
index 5dec208c7393..7e799a84c0a9 100644
--- a/devtools/client/inspector/test/browser_inspector_highlighter-keybinding_04.js
+++ b/devtools/client/inspector/test/browser_inspector_highlighter-keybinding_04.js
@@ -15,7 +15,7 @@ add_task(async function() {
   await startPicker(toolbox);
 
   info("Start using the picker by hovering over nodes");
-  const onHover = toolbox.inspector.nodePicker.once("picker-node-hovered");
+  const onHover = toolbox.inspectorFront.nodePicker.once("picker-node-hovered");
   testActor.synthesizeMouse({
     options: { type: "mousemove" },
     center: true,
@@ -24,7 +24,7 @@ add_task(async function() {
   await onHover;
 
   info("Press escape and wait for the picker to stop");
-  const onPickerStopped = toolbox.inspector.nodePicker.once(
+  const onPickerStopped = toolbox.inspectorFront.nodePicker.once(
     "picker-node-canceled"
   );
   testActor.synthesizeKey({
diff --git a/devtools/client/inspector/test/browser_inspector_highlighter-rulers_01.js b/devtools/client/inspector/test/browser_inspector_highlighter-rulers_01.js
index 6e61397c1eff..814f03d93e37 100644
--- a/devtools/client/inspector/test/browser_inspector_highlighter-rulers_01.js
+++ b/devtools/client/inspector/test/browser_inspector_highlighter-rulers_01.js
@@ -23,7 +23,7 @@ const RULERS_TEXT_STEP = 100;
 
 add_task(async function() {
   const { inspector, testActor } = await openInspectorForURL(TEST_URL);
-  const front = inspector.inspector;
+  const front = inspector.inspectorFront;
 
   const highlighter = await front.getHighlighterByType("RulersHighlighter");
 
diff --git a/devtools/client/inspector/test/browser_inspector_highlighter-rulers_02.js b/devtools/client/inspector/test/browser_inspector_highlighter-rulers_02.js
index 957eaa143bde..7f324432e912 100644
--- a/devtools/client/inspector/test/browser_inspector_highlighter-rulers_02.js
+++ b/devtools/client/inspector/test/browser_inspector_highlighter-rulers_02.js
@@ -15,7 +15,7 @@ const ID = "rulers-highlighter-";
 
 add_task(async function() {
   const { inspector, testActor } = await openInspectorForURL(TEST_URL);
-  const front = inspector.inspector;
+  const front = inspector.inspectorFront;
 
   const highlighter = await front.getHighlighterByType("RulersHighlighter");
 
diff --git a/devtools/client/inspector/test/browser_inspector_highlighter-rulers_03.js b/devtools/client/inspector/test/browser_inspector_highlighter-rulers_03.js
index 6517ffce084b..a387ac8ee49a 100644
--- a/devtools/client/inspector/test/browser_inspector_highlighter-rulers_03.js
+++ b/devtools/client/inspector/test/browser_inspector_highlighter-rulers_03.js
@@ -17,7 +17,7 @@ var { Toolbox } = require("devtools/client/framework/toolbox");
 
 add_task(async function() {
   const { inspector, testActor } = await openInspectorForURL(TEST_URL);
-  const front = inspector.inspector;
+  const front = inspector.inspectorFront;
 
   const highlighter = await front.getHighlighterByType("RulersHighlighter");
 
diff --git a/devtools/client/inspector/test/browser_inspector_highlighter-selector_01.js b/devtools/client/inspector/test/browser_inspector_highlighter-selector_01.js
index e3245cba2382..fe31019f0e31 100644
--- a/devtools/client/inspector/test/browser_inspector_highlighter-selector_01.js
+++ b/devtools/client/inspector/test/browser_inspector_highlighter-selector_01.js
@@ -49,7 +49,7 @@ requestLongerTimeout(5);
 
 add_task(async function() {
   const { inspector, testActor } = await openInspectorForURL(TEST_URL);
-  const front = inspector.inspector;
+  const front = inspector.inspectorFront;
   const highlighter = await front.getHighlighterByType("SelectorHighlighter");
 
   const contextNode = await getNodeFront("body", inspector);
diff --git a/devtools/client/inspector/test/browser_inspector_highlighter-selector_02.js b/devtools/client/inspector/test/browser_inspector_highlighter-selector_02.js
index 182ea5a99c62..a6ebab2dba90 100644
--- a/devtools/client/inspector/test/browser_inspector_highlighter-selector_02.js
+++ b/devtools/client/inspector/test/browser_inspector_highlighter-selector_02.js
@@ -42,7 +42,7 @@ requestLongerTimeout(5);
 
 add_task(async function() {
   const { inspector, testActor } = await openInspectorForURL(TEST_URL);
-  const front = inspector.inspector;
+  const front = inspector.inspectorFront;
   const highlighter = await front.getHighlighterByType("SelectorHighlighter");
 
   for (const { inIframe, selector, containerCount } of TEST_DATA) {
diff --git a/devtools/client/inspector/test/browser_inspector_highlighter-xbl.js b/devtools/client/inspector/test/browser_inspector_highlighter-xbl.js
index 19efb9bf34d5..16e30cfaedcc 100644
--- a/devtools/client/inspector/test/browser_inspector_highlighter-xbl.js
+++ b/devtools/client/inspector/test/browser_inspector_highlighter-xbl.js
@@ -37,6 +37,6 @@ add_task(async function() {
       center: true,
       selector: selector,
     });
-    return inspector.inspector.nodePicker.once("picker-node-hovered");
+    return inspector.inspectorFront.nodePicker.once("picker-node-hovered");
   }
 });
diff --git a/devtools/client/inspector/test/browser_inspector_iframe-navigation.js b/devtools/client/inspector/test/browser_inspector_iframe-navigation.js
index 4d55720e4246..4d3433c9a50d 100644
--- a/devtools/client/inspector/test/browser_inspector_iframe-navigation.js
+++ b/devtools/client/inspector/test/browser_inspector_iframe-navigation.js
@@ -40,5 +40,5 @@ add_task(async function() {
   ok(isVisible, "Inspector is highlighting after iframe nav.");
 
   info("Stopping element picker.");
-  await toolbox.inspector.nodePicker.stop();
+  await toolbox.inspectorFront.nodePicker.stop();
 });
diff --git a/devtools/client/inspector/test/browser_inspector_picker-stop-on-tool-change.js b/devtools/client/inspector/test/browser_inspector_picker-stop-on-tool-change.js
index 201697cf5c69..7a6fe371f25c 100644
--- a/devtools/client/inspector/test/browser_inspector_picker-stop-on-tool-change.js
+++ b/devtools/client/inspector/test/browser_inspector_picker-stop-on-tool-change.js
@@ -13,7 +13,9 @@ const TEST_URI =
 
 add_task(async function() {
   const { toolbox } = await openInspectorForURL(TEST_URI);
-  const pickerStopped = toolbox.inspector.nodePicker.once("picker-stopped");
+  const pickerStopped = toolbox.inspectorFront.nodePicker.once(
+    "picker-stopped"
+  );
 
   info("Starting the inspector picker");
   await startPicker(toolbox);
diff --git a/devtools/client/inspector/test/browser_inspector_switch-to-inspector-on-pick.js b/devtools/client/inspector/test/browser_inspector_switch-to-inspector-on-pick.js
index 8c5b1f0cb443..5761fdb95310 100644
--- a/devtools/client/inspector/test/browser_inspector_switch-to-inspector-on-pick.js
+++ b/devtools/client/inspector/test/browser_inspector_switch-to-inspector-on-pick.js
@@ -67,7 +67,7 @@ add_task(async function() {
   await startPickerAndAssertSwitchToInspector(toolbox);
 
   info("Stoppping element picker.");
-  await toolbox.inspector.nodePicker.stop();
+  await toolbox.inspectorFront.nodePicker.stop();
 
   checkResults();
 });
diff --git a/devtools/client/inspector/test/head.js b/devtools/client/inspector/test/head.js
index 1c407e8b44a6..a1e714836b8d 100644
--- a/devtools/client/inspector/test/head.js
+++ b/devtools/client/inspector/test/head.js
@@ -76,7 +76,7 @@ var navigateTo = async function(inspector, url) {
 var startPicker = async function(toolbox, skipFocus) {
   info("Start the element picker");
   toolbox.win.focus();
-  await toolbox.inspector.nodePicker.start();
+  await toolbox.inspectorFront.nodePicker.start();
   if (!skipFocus) {
     // By default make sure the content window is focused since the picker may not focus
     // the content window by default.
@@ -129,7 +129,9 @@ function pickElement(inspector, testActor, selector, x, y) {
  */
 function hoverElement(inspector, testActor, selector, x, y) {
   info("Waiting for element " + selector + " to be hovered");
-  const onHovered = inspector.inspector.nodePicker.once("picker-node-hovered");
+  const onHovered = inspector.inspectorFront.nodePicker.once(
+    "picker-node-hovered"
+  );
   testActor.synthesizeMouse({ selector, x, y, options: { type: "mousemove" } });
   return onHovered;
 }
@@ -504,7 +506,7 @@ async function poll(check, desc, attempts = 10, timeBetweenAttempts = 200) {
  */
 const getHighlighterHelperFor = type =>
   async function({ inspector, testActor }) {
-    const front = inspector.inspector;
+    const front = inspector.inspectorFront;
     const highlighter = await front.getHighlighterByType(type);
 
     let prefix = "";
diff --git a/devtools/client/shared/components/reps/reps.js b/devtools/client/shared/components/reps/reps.js
index 51bd8a061926..75d3a6295b21 100644
--- a/devtools/client/shared/components/reps/reps.js
+++ b/devtools/client/shared/components/reps/reps.js
@@ -588,7 +588,7 @@ class Tree extends Component {
     super(props);
 
     this.state = {
-      seen: new Set()
+      autoExpanded: new Set()
     };
 
     this.treeRef = _react2.default.createRef();
@@ -632,26 +632,36 @@ class Tree extends Component {
   }
 
   _autoExpand() {
-    const { autoExpandDepth, autoExpandNodeChildrenLimit } = this.props;
-    if (!autoExpandDepth) {
+    const {
+      autoExpandDepth,
+      autoExpandNodeChildrenLimit,
+      initiallyExpanded
+    } = this.props;
+
+    if (!autoExpandDepth && !initiallyExpanded) {
       return;
     }
 
     // Automatically expand the first autoExpandDepth levels for new items. Do
     // not use the usual DFS infrastructure because we don't want to ignore
-    // collapsed nodes.
+    // collapsed nodes. Any initially expanded items will be expanded regardless
+    // of how deep they are.
     const autoExpand = (item, currentDepth) => {
-      if (currentDepth >= autoExpandDepth || this.state.seen.has(item)) {
+      const initial = initiallyExpanded && initiallyExpanded(item);
+
+      if (!initial && currentDepth >= autoExpandDepth) {
         return;
       }
 
       const children = this.props.getChildren(item);
-      if (autoExpandNodeChildrenLimit && children.length > autoExpandNodeChildrenLimit) {
+      if (!initial && autoExpandNodeChildrenLimit && children.length > autoExpandNodeChildrenLimit) {
         return;
       }
 
-      this.props.onExpand(item);
-      this.state.seen.add(item);
+      if (!this.state.autoExpanded.has(item)) {
+        this.props.onExpand(item);
+        this.state.autoExpanded.add(item);
+      }
 
       const length = children.length;
       for (let i = 0; i < length; i++) {
@@ -667,6 +677,14 @@ class Tree extends Component {
       }
     } else if (length != 0) {
       autoExpand(roots[0], 0);
+
+      if (initiallyExpanded) {
+        for (let i = 1; i < length; i++) {
+          if (initiallyExpanded(roots[i])) {
+            autoExpand(roots[i], 0);
+          }
+        }
+      }
     }
   }
 
@@ -7001,6 +7019,7 @@ class ObjectInspector extends Component {
       nodeExpand,
       nodeCollapse,
       recordTelemetryEvent,
+      setExpanded,
       roots
     } = this.props;
 
@@ -7013,6 +7032,10 @@ class ObjectInspector extends Component {
     } else {
       nodeCollapse(item);
     }
+
+    if (setExpanded) {
+      setExpanded(item, expand);
+    }
   }
 
   focusItem(item) {
@@ -7052,6 +7075,7 @@ class ObjectInspector extends Component {
     const {
       autoExpandAll = true,
       autoExpandDepth = 1,
+      initiallyExpanded,
       focusable = true,
       disableWrap = false,
       expandedPaths,
@@ -7067,6 +7091,7 @@ class ObjectInspector extends Component {
 
       autoExpandAll,
       autoExpandDepth,
+      initiallyExpanded,
 
       isExpanded: item => expandedPaths && expandedPaths.has(item.path),
       isExpandable: this.isNodeExpandable,
@@ -7212,8 +7237,7 @@ function rootsChanged(props) {
 function releaseActors(state, client) {
   const actors = getActors(state);
   for (const actor of actors) {
-    // Ignore release failure, since the object actor may have been already GC.
-    client.releaseActor(actor).catch(() => {});
+    client.releaseActor(actor);
   }
 }
 
@@ -7586,4 +7610,4 @@ var __WEBPACK_AMD_DEFINE_ARRAY__, __WEBPACK_AMD_DEFINE_RESULT__;/*!
 /***/ })
 
 /******/ });
-});
+});
\ No newline at end of file
diff --git a/devtools/client/shared/widgets/tooltip/SwatchColorPickerTooltip.js b/devtools/client/shared/widgets/tooltip/SwatchColorPickerTooltip.js
index 98ef934f145e..87e8ea77a681 100644
--- a/devtools/client/shared/widgets/tooltip/SwatchColorPickerTooltip.js
+++ b/devtools/client/shared/widgets/tooltip/SwatchColorPickerTooltip.js
@@ -236,34 +236,34 @@ class SwatchColorPickerTooltip extends SwatchBasedEditorTooltip {
   }
 
   _openEyeDropper() {
-    const { inspector, toolbox, telemetry } = this.inspector;
+    const { inspectorFront, toolbox, telemetry } = this.inspector;
 
     telemetry
       .getHistogramById(TELEMETRY_PICKER_EYEDROPPER_OPEN_COUNT)
       .add(true);
 
     // cancelling picker(if it is already selected) on opening eye-dropper
-    inspector.nodePicker.cancel();
+    inspectorFront.nodePicker.cancel();
 
     // pickColorFromPage will focus the content document. If the devtools are in a
     // separate window, the colorpicker tooltip will be closed before pickColorFromPage
     // resolves. Flip the flag early to avoid issues with onTooltipHidden().
     this.eyedropperOpen = true;
 
-    inspector.pickColorFromPage({ copyOnSelect: false }).then(() => {
+    inspectorFront.pickColorFromPage({ copyOnSelect: false }).then(() => {
       // close the colorpicker tooltip so that only the eyedropper is open.
       this.hide();
 
       this.tooltip.emit("eyedropper-opened");
     }, console.error);
 
-    inspector.once("color-picked", color => {
+    inspectorFront.once("color-picked", color => {
       toolbox.win.focus();
       this._selectColor(color);
       this._onEyeDropperDone();
     });
 
-    inspector.once("color-pick-canceled", () => {
+    inspectorFront.once("color-pick-canceled", () => {
       this._onEyeDropperDone();
     });
   }
diff --git a/devtools/client/styleeditor/StyleEditorUI.jsm b/devtools/client/styleeditor/StyleEditorUI.jsm
index 70bc261e60f3..9d7ddfb1b986 100644
--- a/devtools/client/styleeditor/StyleEditorUI.jsm
+++ b/devtools/client/styleeditor/StyleEditorUI.jsm
@@ -158,7 +158,7 @@ StyleEditorUI.prototype = {
     this._walker = this._toolbox.walker;
 
     try {
-      this._highlighter = await this._toolbox.inspector.getHighlighterByType(
+      this._highlighter = await this._toolbox.inspectorFront.getHighlighterByType(
         SELECTOR_HIGHLIGHTER_TYPE
       );
     } catch (e) {
diff --git a/devtools/client/webconsole/test/mochitest/head.js b/devtools/client/webconsole/test/mochitest/head.js
index 6a1b708de0c1..3cf0078a8d34 100644
--- a/devtools/client/webconsole/test/mochitest/head.js
+++ b/devtools/client/webconsole/test/mochitest/head.js
@@ -1139,7 +1139,7 @@ function isReverseSearchInputFocused(hud) {
  */
 async function selectNodeWithPicker(toolbox, testActor, selector) {
   const inspector = toolbox.getPanel("inspector");
-  const inspectorFront = inspector.inspector;
+  const inspectorFront = inspector.inspectorFront;
 
   const onPickerStarted = inspectorFront.nodePicker.once("picker-started");
   inspectorFront.nodePicker.start();
diff --git a/devtools/client/webreplay/mochitest/browser_rr_inspector-02.js b/devtools/client/webreplay/mochitest/browser_rr_inspector-02.js
index f4a1050884c6..0df95da0ee99 100644
--- a/devtools/client/webreplay/mochitest/browser_rr_inspector-02.js
+++ b/devtools/client/webreplay/mochitest/browser_rr_inspector-02.js
@@ -28,7 +28,7 @@ add_task(async function() {
 
   info("Waiting for element picker to become active.");
   toolbox.win.focus();
-  await toolbox.inspector.nodePicker.start();
+  await toolbox.inspectorFront.nodePicker.start();
 
   info("Moving mouse over div.");
   await moveMouseOver("#maindiv", 1, 1);
@@ -50,6 +50,6 @@ add_task(async function() {
       y,
       options: { type: "mousemove" },
     });
-    return inspector.inspector.nodePicker.once("picker-node-hovered");
+    return inspector.inspectorFront.nodePicker.once("picker-node-hovered");
   }
 });
diff --git a/devtools/server/actors/replay/control.js b/devtools/server/actors/replay/control.js
index 847289cb5d69..dc9912232b51 100644
--- a/devtools/server/actors/replay/control.js
+++ b/devtools/server/actors/replay/control.js
@@ -111,6 +111,9 @@ function ChildProcess(id, recording) {
   // The last point we paused at.
   this.lastPausePoint = null;
 
+  // Last reported memory usage for this child.
+  this.lastMemoryUsage = null;
+
   // Manifests which this child needs to send asynchronously.
   this.asyncManifests = [];
 
@@ -168,19 +171,25 @@ ChildProcess.prototype = {
     this.paused = false;
     this.manifest = manifest;
 
-    dumpv(`SendManifest #${this.id} ${JSON.stringify(manifest.contents)}`);
+    dumpv(`SendManifest #${this.id} ${stringify(manifest.contents)}`);
     RecordReplayControl.sendManifest(this.id, manifest.contents);
   },
 
   // Called when the child's current manifest finishes.
   manifestFinished(response) {
     assert(!this.paused);
-    if (response && response.point) {
-      this.lastPausePoint = response.point;
+    if (response) {
+      if (response.point) {
+        this.lastPausePoint = response.point;
+      }
+      if (response.memoryUsage) {
+        this.lastMemoryUsage = response.memoryUsage;
+      }
     }
     this.paused = true;
     this.manifest.onFinished(response);
     this.manifest = null;
+    maybeDumpStatistics();
   },
 
   // Block until this child is paused. If maybeCreateCheckpoint is specified
@@ -346,6 +355,15 @@ function CheckpointInfo() {
   // If the checkpoint is saved, the replaying child responsible for saving it
   // and scanning the region up to the next saved checkpoint.
   this.owner = null;
+
+  // If the checkpoint is saved, the time it was assigned an owner.
+  this.assignTime = null;
+
+  // If the checkpoint is saved and scanned, the time it finished being scanned.
+  this.scanTime = null;
+
+  // If the checkpoint is saved and scanned, the duration of the scan.
+  this.scanDuration = null;
 }
 
 function getCheckpointInfo(id) {
@@ -364,18 +382,22 @@ function timeSinceCheckpoint(id) {
   return time;
 }
 
+// How much execution time is captured by a saved checkpoint.
+function timeForSavedCheckpoint(id) {
+  const next = nextSavedCheckpoint(id);
+  let time = 0;
+  for (let i = id; i < next; i++) {
+    time += gCheckpoints[i].duration;
+  }
+  return time;
+}
+
 // The checkpoint up to which the recording runs.
 let gLastFlushCheckpoint = InvalidCheckpointId;
 
-// The last saved checkpoint.
-let gLastSavedCheckpoint = FirstCheckpointId;
-
 // How often we want to flush the recording.
 const FlushMs = 0.5 * 1000;
 
-// How often we want to save a checkpoint.
-const SavedCheckpointMs = 0.25 * 1000;
-
 function addSavedCheckpoint(checkpoint) {
   if (getCheckpointInfo(checkpoint).owner) {
     return;
@@ -383,26 +405,16 @@ function addSavedCheckpoint(checkpoint) {
 
   const owner = pickReplayingChild();
   getCheckpointInfo(checkpoint).owner = owner;
+  getCheckpointInfo(checkpoint).assignTime = Date.now();
   owner.addSavedCheckpoint(checkpoint);
-  gLastSavedCheckpoint = checkpoint;
 }
 
 function addCheckpoint(checkpoint, duration) {
   assert(!getCheckpointInfo(checkpoint).duration);
   getCheckpointInfo(checkpoint).duration = duration;
-
-  // Mark saved checkpoints as required, unless we haven't spawned any replaying
-  // children yet.
-  if (
-    timeSinceCheckpoint(gLastSavedCheckpoint) >= SavedCheckpointMs &&
-    gReplayingChildren.length > 0
-  ) {
-    addSavedCheckpoint(checkpoint + 1);
-  }
 }
 
 function ownerChild(checkpoint) {
-  assert(checkpoint <= gLastSavedCheckpoint);
   while (!getCheckpointInfo(checkpoint).owner) {
     checkpoint--;
   }
@@ -559,7 +571,14 @@ function scanRecording(checkpoint) {
           needSaveCheckpoints: child.flushNeedSaveCheckpoints(),
         };
       },
-      onFinished: child => child.scannedCheckpoints.add(checkpoint),
+      onFinished(child, { duration }) {
+        child.scannedCheckpoints.add(checkpoint);
+        const info = getCheckpointInfo(checkpoint);
+        if (!info.scanTime) {
+          info.scanTime = Date.now();
+          info.scanDuration = duration;
+        }
+      },
     },
     checkpointExecutionPoint(checkpoint)
   );
@@ -1006,8 +1025,8 @@ function handleResumeManifestResponse({
     consoleMessages.forEach(msg => gDebugger.onConsoleMessage(msg));
   }
 
-  if (gDebugger && gDebugger.onNewScript) {
-    scripts.forEach(script => gDebugger.onNewScript(script));
+  if (gDebugger) {
+    scripts.forEach(script => gDebugger._onNewScript(script));
   }
 }
 
@@ -1071,8 +1090,8 @@ function ensureFlushed() {
     spawnReplayingChildren();
   }
 
-  // Checkpoints where the recording was flushed to disk are always saved.
-  // This allows the recording to be scanned as soon as it has been flushed.
+  // Checkpoints where the recording was flushed to disk are saved. This allows
+  // the recording to be scanned as soon as it has been flushed.
   addSavedCheckpoint(gLastFlushCheckpoint);
 
   // Flushing creates a new region of the recording for replaying children
@@ -1156,7 +1175,7 @@ function Initialize(recordingChildId) {
 // eslint-disable-next-line no-unused-vars
 function ManifestFinished(id, response) {
   try {
-    dumpv(`ManifestFinished #${id} ${JSON.stringify(response)}`);
+    dumpv(`ManifestFinished #${id} ${stringify(response)}`);
     lookupChild(id).manifestFinished(response);
   } catch (e) {
     dump(`ERROR: ManifestFinished threw exception: ${e} ${e.stack}\n`);
@@ -1331,6 +1350,75 @@ const gControl = {
   },
 };
 
+///////////////////////////////////////////////////////////////////////////////
+// Statistics
+///////////////////////////////////////////////////////////////////////////////
+
+let lastDumpTime = Date.now();
+
+function maybeDumpStatistics() {
+  const now = Date.now();
+  if (now - lastDumpTime < 5000) {
+    return;
+  }
+  lastDumpTime = now;
+
+  let delayTotal = 0;
+  let unscannedTotal = 0;
+  let timeTotal = 0;
+  let scanDurationTotal = 0;
+
+  forSavedCheckpointsInRange(
+    FirstCheckpointId,
+    gLastFlushCheckpoint,
+    checkpoint => {
+      const checkpointTime = timeForSavedCheckpoint(checkpoint);
+      const info = getCheckpointInfo(checkpoint);
+
+      timeTotal += checkpointTime;
+      if (info.scanTime) {
+        delayTotal += checkpointTime * (info.scanTime - info.assignTime);
+        scanDurationTotal += info.scanDuration;
+      } else {
+        unscannedTotal += checkpointTime;
+      }
+    }
+  );
+
+  const memoryUsage = [];
+  let totalSaved = 0;
+
+  for (const child of gReplayingChildren) {
+    if (!child) {
+      continue;
+    }
+    totalSaved += child.savedCheckpoints.size;
+    if (!child.lastMemoryUsage) {
+      continue;
+    }
+    for (const [name, value] of Object.entries(child.lastMemoryUsage)) {
+      if (!memoryUsage[name]) {
+        memoryUsage[name] = 0;
+      }
+      memoryUsage[name] += value;
+    }
+  }
+
+  const delay = delayTotal / timeTotal;
+  const overhead = scanDurationTotal / (timeTotal - unscannedTotal);
+
+  dumpv(`Statistics:`);
+  dumpv(`Total recording time: ${timeTotal}`);
+  dumpv(`Unscanned fraction: ${unscannedTotal / timeTotal}`);
+  dumpv(`Average scan delay: ${delay}`);
+  dumpv(`Average scanning overhead: ${overhead}`);
+
+  dumpv(`Saved checkpoints: ${totalSaved}`);
+  for (const [name, value] of Object.entries(memoryUsage)) {
+    dumpv(`Memory ${name}: ${value}`);
+  }
+}
+
 ///////////////////////////////////////////////////////////////////////////////
 // Utilities
 ///////////////////////////////////////////////////////////////////////////////
@@ -1341,8 +1429,15 @@ function ConnectDebugger(dbg) {
   dbg._control = gControl;
 }
 
+const startTime = Date.now();
+
+// eslint-disable-next-line no-unused-vars
+function currentTime() {
+  return (((Date.now() - startTime) / 10) | 0) / 100;
+}
+
 function dumpv(str) {
-  //dump(`[ReplayControl] ${str}\n`);
+  //dump(`[ReplayControl ${currentTime()}] ${str}\n`);
 }
 
 function assert(v) {
@@ -1357,6 +1452,14 @@ function ThrowError(msg) {
   throw error;
 }
 
+function stringify(object) {
+  const str = JSON.stringify(object);
+  if (str && str.length >= 4096) {
+    return `${str.substr(0, 4096)} TRIMMED ${str.length}`;
+  }
+  return str;
+}
+
 // eslint-disable-next-line no-unused-vars
 var EXPORTED_SYMBOLS = [
   "Initialize",
diff --git a/devtools/server/actors/replay/debugger.js b/devtools/server/actors/replay/debugger.js
index 836670cc8309..c02acbaad475 100644
--- a/devtools/server/actors/replay/debugger.js
+++ b/devtools/server/actors/replay/debugger.js
@@ -123,9 +123,7 @@ ReplayDebugger.prototype = {
   },
 
   _processResponse(request, response, divergeResponse) {
-    dumpv(
-      `SendRequest: ${JSON.stringify(request)} -> ${JSON.stringify(response)}`
-    );
+    dumpv(`SendRequest: ${stringify(request)} -> ${stringify(response)}`);
     if (response.exception) {
       ThrowError(response.exception);
     }
@@ -324,7 +322,7 @@ ReplayDebugger.prototype = {
   replayPushThreadPause() {
     // The thread has paused so that the user can interact with it. The child
     // will stay paused until this thread-wide pause has been popped.
-    assert(this._paused);
+    this._ensurePaused();
     assert(!this._resumeCallback);
     if (++this._threadPauseCount == 1) {
       // There is no preferred direction of travel after an explicit pause.
@@ -363,7 +361,8 @@ ReplayDebugger.prototype = {
   },
 
   _performResume() {
-    assert(this._paused && !this._threadPauseCount);
+    this._ensurePaused();
+    assert(!this._threadPauseCount);
     if (this._resumeCallback && !this._threadPauseCount) {
       const callback = this._resumeCallback;
       this._invalidateAfterUnpause();
@@ -528,11 +527,11 @@ ReplayDebugger.prototype = {
     return data.map(script => this._addScript(script));
   },
 
-  findAllConsoleMessages() {
-    const messages = this._sendRequestMainChild({
-      type: "findConsoleMessages",
-    });
-    return messages.map(this._convertConsoleMessage.bind(this));
+  _onNewScript(data) {
+    if (this.onNewScript) {
+      const script = this._addScript(data);
+      this.onNewScript(script);
+    }
   },
 
   /////////////////////////////////////////////////////////
@@ -544,7 +543,9 @@ ReplayDebugger.prototype = {
     if (source) {
       return source;
     }
-    return this._addSource(this._sendRequest({ type: "getSource", id }));
+    return this._addSource(
+      this._sendRequestMainChild({ type: "getSource", id })
+    );
   },
 
   _addSource(data) {
@@ -692,6 +693,13 @@ ReplayDebugger.prototype = {
     return message;
   },
 
+  findAllConsoleMessages() {
+    const messages = this._sendRequestMainChild({
+      type: "findConsoleMessages",
+    });
+    return messages.map(this._convertConsoleMessage.bind(this));
+  },
+
   /////////////////////////////////////////////////////////
   // Handlers
   /////////////////////////////////////////////////////////
@@ -1362,4 +1370,12 @@ function isNonNullObject(obj) {
   return obj && (typeof obj == "object" || typeof obj == "function");
 }
 
+function stringify(object) {
+  const str = JSON.stringify(object);
+  if (str.length >= 4096) {
+    return `${str.substr(0, 4096)} TRIMMED ${str.length}`;
+  }
+  return str;
+}
+
 module.exports = ReplayDebugger;
diff --git a/devtools/server/actors/replay/replay.js b/devtools/server/actors/replay/replay.js
index a3b1aed43617..5976fef2e5f7 100644
--- a/devtools/server/actors/replay/replay.js
+++ b/devtools/server/actors/replay/replay.js
@@ -68,7 +68,9 @@ dbg.onNewGlobalObject = function(global) {
 // Utilities
 ///////////////////////////////////////////////////////////////////////////////
 
-const dump = RecordReplayControl.dump;
+const dump = str => {
+  RecordReplayControl.dump(`[Child #${RecordReplayControl.childId()}]: ${str}`);
+};
 
 function assert(v) {
   if (!v) {
@@ -143,6 +145,24 @@ function isNonNullObject(obj) {
   return obj && (typeof obj == "object" || typeof obj == "function");
 }
 
+function getMemoryUsage() {
+  const memoryKinds = {
+    Generic: [1],
+    Snapshots: [2, 3, 4, 5, 6, 7],
+    ScriptHits: [8],
+  };
+
+  const rv = {};
+  for (const [name, kinds] of Object.entries(memoryKinds)) {
+    let total = 0;
+    kinds.forEach(kind => {
+      total += RecordReplayControl.memoryUsage(kind);
+    });
+    rv[name] = total;
+  }
+  return rv;
+}
+
 ///////////////////////////////////////////////////////////////////////////////
 // Persistent Script State
 ///////////////////////////////////////////////////////////////////////////////
@@ -637,16 +657,16 @@ function ClearPausedState() {
 // The manifest that is currently being processed.
 let gManifest;
 
-// When processing "resume" manifests this tracks the execution time when we
-// started execution from the initial checkpoint.
-let gTimeWhenResuming;
+// When processing certain manifests this tracks the execution time when the
+// manifest started executing.
+let gManifestStartTime;
 
 // Handlers that run when a manifest is first received. This must be specified
 // for all manifests.
 const gManifestStartHandlers = {
   resume({ breakpoints }) {
     RecordReplayControl.resumeExecution();
-    gTimeWhenResuming = RecordReplayControl.currentExecutionTime();
+    gManifestStartTime = RecordReplayControl.currentExecutionTime();
     breakpoints.forEach(ensurePositionHandler);
   },
 
@@ -663,6 +683,7 @@ const gManifestStartHandlers = {
   },
 
   scanRecording(manifest) {
+    gManifestStartTime = RecordReplayControl.currentExecutionTime();
     gManifestStartHandlers.runToPoint(manifest);
   },
 
@@ -801,7 +822,7 @@ const gManifestFinishedAfterCheckpointHandlers = {
   resume(_, point) {
     RecordReplayControl.manifestFinished({
       point,
-      duration: RecordReplayControl.currentExecutionTime() - gTimeWhenResuming,
+      duration: RecordReplayControl.currentExecutionTime() - gManifestStartTime,
       consoleMessages: gNewConsoleMessages,
       scripts: gNewScripts,
     });
@@ -810,6 +831,7 @@ const gManifestFinishedAfterCheckpointHandlers = {
   },
 
   runToPoint({ endpoint }, point) {
+    assert(endpoint.checkpoint >= point.checkpoint);
     if (!endpoint.position && point.checkpoint == endpoint.checkpoint) {
       RecordReplayControl.manifestFinished({ point });
     }
@@ -817,7 +839,13 @@ const gManifestFinishedAfterCheckpointHandlers = {
 
   scanRecording({ endpoint }, point) {
     if (point.checkpoint == endpoint) {
-      RecordReplayControl.manifestFinished({ point });
+      const duration =
+        RecordReplayControl.currentExecutionTime() - gManifestStartTime;
+      RecordReplayControl.manifestFinished({
+        point,
+        duration,
+        memoryUsage: getMemoryUsage(),
+      });
     }
   },
 };
@@ -892,6 +920,7 @@ let gFrameStepsFrameIndex = 0;
 // This must be specified for any manifest that uses ensurePositionHandler.
 const gManifestPositionHandlers = {
   resume(manifest, point) {
+    clearPositionHandlers();
     RecordReplayControl.manifestFinished({
       point,
       consoleMessages: gNewConsoleMessages,
@@ -1066,7 +1095,7 @@ function getObjectData(id) {
       optimizedOut: object.optimizedOut,
     };
   }
-  throwError("Unknown object kind");
+  throwError(`Unknown object kind: ${object}`);
 }
 
 function getObjectProperties(object) {
@@ -1249,6 +1278,7 @@ function getPauseData() {
     const names = getEnvironmentNames(env);
     rv.environments[id] = { data, names };
 
+    addObject(data.callee);
     addEnvironment(data.parent);
   }
 
diff --git a/toolkit/recordreplay/MemorySnapshot.cpp b/toolkit/recordreplay/MemorySnapshot.cpp
index 6a625d15f50c..1ad8db973005 100644
--- a/toolkit/recordreplay/MemorySnapshot.cpp
+++ b/toolkit/recordreplay/MemorySnapshot.cpp
@@ -1312,5 +1312,12 @@ void RestoreMemoryToLastSavedDiffCheckpoint() {
   gMemoryInfo->mSnapshotThreadsShouldRestore.ActivateEnd();
 }
 
+size_t GetMemoryUsage(MemoryKind aKind) {
+  if (!gMemoryInfo) {
+    return 0;
+  }
+  return gMemoryInfo->mMemoryBalance[(size_t)aKind];
+}
+
 }  // namespace recordreplay
 }  // namespace mozilla
diff --git a/toolkit/recordreplay/MemorySnapshot.h b/toolkit/recordreplay/MemorySnapshot.h
index d9a8d4d0e194..03f63e42a072 100644
--- a/toolkit/recordreplay/MemorySnapshot.h
+++ b/toolkit/recordreplay/MemorySnapshot.h
@@ -126,6 +126,9 @@ void MemoryMove(void* aDst, const void* aSrc, size_t aSize);
 // dynamic code loading.
 void MemoryZero(void* aDst, size_t aSize);
 
+// Get the amount of allocated memory used by data of the specified kind.
+size_t GetMemoryUsage(MemoryKind aKind);
+
 }  // namespace recordreplay
 }  // namespace mozilla
 
diff --git a/toolkit/recordreplay/ProcessRedirectDarwin.cpp b/toolkit/recordreplay/ProcessRedirectDarwin.cpp
index b02cc61f3027..e88b6e1d2997 100644
--- a/toolkit/recordreplay/ProcessRedirectDarwin.cpp
+++ b/toolkit/recordreplay/ProcessRedirectDarwin.cpp
@@ -340,10 +340,30 @@ static void MM_CFTypeOutputArg(MiddlemanCallContext& aCx) {
   MM_CFTypeOutput(aCx, arg, /* aOwnsReference = */ false);
 }
 
+static void SendMessageToObject(const void* aObject, const char* aMessage) {
+  CallArguments arguments;
+  arguments.Arg<0, const void*>() = aObject;
+  arguments.Arg<1, SEL>() = sel_registerName(aMessage);
+  RecordReplayInvokeCall(gOriginal_objc_msgSend, &arguments);
+}
+
 // For APIs whose result will be released by the middleman's autorelease pool.
 static void MM_AutoreleaseCFTypeRval(MiddlemanCallContext& aCx) {
   auto& rval = aCx.mArguments->Rval<const void*>();
   MM_SystemOutput(aCx, &rval);
+
+  if (rval) {
+    switch (aCx.mPhase) {
+      case MiddlemanCallPhase::MiddlemanOutput:
+        SendMessageToObject(rval, "retain");
+        break;
+      case MiddlemanCallPhase::MiddlemanRelease:
+        SendMessageToObject(rval, "autorelease");
+        break;
+      default:
+        break;
+    }
+  }
 }
 
 // For functions which have an input CFType value and also have side effects on
diff --git a/toolkit/recordreplay/ipc/ChildIPC.cpp b/toolkit/recordreplay/ipc/ChildIPC.cpp
index 871f7286b0ca..22d313b32904 100644
--- a/toolkit/recordreplay/ipc/ChildIPC.cpp
+++ b/toolkit/recordreplay/ipc/ChildIPC.cpp
@@ -345,6 +345,10 @@ void ReportFatalError(const Maybe<MinidumpInfo>& aMinidump, const char* aFormat,
   Thread::WaitForeverNoIdle();
 }
 
+size_t GetId() {
+  return gChannel->GetId();
+}
+
 ///////////////////////////////////////////////////////////////////////////////
 // Vsyncs
 ///////////////////////////////////////////////////////////////////////////////
diff --git a/toolkit/recordreplay/ipc/ChildInternal.h b/toolkit/recordreplay/ipc/ChildInternal.h
index e99fa84b7b5f..62282a614b66 100644
--- a/toolkit/recordreplay/ipc/ChildInternal.h
+++ b/toolkit/recordreplay/ipc/ChildInternal.h
@@ -39,6 +39,9 @@ struct MinidumpInfo {
 void ReportFatalError(const Maybe<MinidumpInfo>& aMinidumpInfo,
                       const char* aFormat, ...);
 
+// Get the unique ID of this child.
+size_t GetId();
+
 // Monitor used for various synchronization tasks.
 extern Monitor* gMonitor;
 
diff --git a/toolkit/recordreplay/ipc/ChildProcess.cpp b/toolkit/recordreplay/ipc/ChildProcess.cpp
index 812f5e3cb74a..d4df5df870fa 100644
--- a/toolkit/recordreplay/ipc/ChildProcess.cpp
+++ b/toolkit/recordreplay/ipc/ChildProcess.cpp
@@ -264,13 +264,17 @@ void ChildProcessInfo::WaitUntilPaused() {
 
   bool sentTerminateMessage = false;
   while (true) {
-    MonitorAutoLock lock(*gMonitor);
+    Maybe<MonitorAutoLock> lock;
+    lock.emplace(*gMonitor);
+
+    MaybeHandlePendingSyncMessage();
 
     // Search for the first message received from this process.
     ChildProcessInfo* process = this;
     Message::UniquePtr msg = ExtractChildMessage(&process);
 
     if (msg) {
+      lock.reset();
       OnIncomingMessage(*msg);
       if (IsPaused()) {
         return;
diff --git a/toolkit/recordreplay/ipc/JSControl.cpp b/toolkit/recordreplay/ipc/JSControl.cpp
index c5fd3a562f8a..8e9bb34e2c9c 100644
--- a/toolkit/recordreplay/ipc/JSControl.cpp
+++ b/toolkit/recordreplay/ipc/JSControl.cpp
@@ -565,6 +565,13 @@ static bool FetchContent(JSContext* aCx, HandleString aURL,
 // Recording/Replaying Methods
 ///////////////////////////////////////////////////////////////////////////////
 
+static bool RecordReplay_ChildId(JSContext* aCx, unsigned aArgc, Value* aVp) {
+  CallArgs args = CallArgsFromVp(aArgc, aVp);
+
+  args.rval().setInt32(child::GetId());
+  return true;
+}
+
 static bool RecordReplay_AreThreadEventsDisallowed(JSContext* aCx,
                                                    unsigned aArgc, Value* aVp) {
   CallArgs args = CallArgsFromVp(aArgc, aVp);
@@ -971,6 +978,26 @@ static bool RecordReplay_Repaint(JSContext* aCx, unsigned aArgc, Value* aVp) {
   return true;
 }
 
+static bool RecordReplay_MemoryUsage(JSContext* aCx, unsigned aArgc,
+                                     Value* aVp) {
+  CallArgs args = CallArgsFromVp(aArgc, aVp);
+
+  if (!args.get(0).isNumber()) {
+    JS_ReportErrorASCII(aCx, "Bad memory kind");
+    return false;
+  }
+
+  size_t kind = args.get(0).toNumber();
+
+  if (kind >= (size_t) MemoryKind::Count) {
+    JS_ReportErrorASCII(aCx, "Memory kind out of range");
+    return false;
+  }
+
+  args.rval().setDouble(GetMemoryUsage((MemoryKind) kind));
+  return true;
+}
+
 static bool RecordReplay_Dump(JSContext* aCx, unsigned aArgc, Value* aVp) {
   // This method is an alternative to dump() that can be used in places where
   // thread events are disallowed.
@@ -1007,6 +1034,7 @@ static const JSFunctionSpec gMiddlemanMethods[] = {
     JS_FS_END};
 
 static const JSFunctionSpec gRecordReplayMethods[] = {
+    JS_FN("childId", RecordReplay_ChildId, 0, 0),
     JS_FN("areThreadEventsDisallowed", RecordReplay_AreThreadEventsDisallowed,
           0, 0),
     JS_FN("divergeFromRecording", RecordReplay_DivergeFromRecording, 0, 0),
@@ -1025,6 +1053,7 @@ static const JSFunctionSpec gRecordReplayMethods[] = {
     JS_FN("findScriptHits", RecordReplay_FindScriptHits, 2, 0),
     JS_FN("getContent", RecordReplay_GetContent, 1, 0),
     JS_FN("repaint", RecordReplay_Repaint, 0, 0),
+    JS_FN("memoryUsage", RecordReplay_MemoryUsage, 0, 0),
     JS_FN("dump", RecordReplay_Dump, 1, 0),
     JS_FS_END};
 
diff --git a/toolkit/recordreplay/ipc/ParentForwarding.cpp b/toolkit/recordreplay/ipc/ParentForwarding.cpp
index 5c78f3b9623c..3545de9239f9 100644
--- a/toolkit/recordreplay/ipc/ParentForwarding.cpp
+++ b/toolkit/recordreplay/ipc/ParentForwarding.cpp
@@ -260,89 +260,83 @@ class MiddlemanProtocol : public ipc::IToplevelProtocol {
     return MsgProcessed;
   }
 
-  static void ForwardMessageSync(MiddlemanProtocol* aProtocol,
-                                 Message* aMessage, Message** aReply) {
-    PrintSpew("ForwardSyncMsg %s\n",
-              IPC::StringFromIPCMessageType(aMessage->type()));
+  Message* mSyncMessage = nullptr;
+  Message* mSyncMessageReply = nullptr;
+  bool mSyncMessageIsCall = false;
 
-    MOZ_RELEASE_ASSERT(!*aReply);
-    Message* nReply = new Message();
-    if (!aProtocol->GetIPCChannel()->Send(aMessage, nReply)) {
-      MOZ_RELEASE_ASSERT(aProtocol->mSide == ipc::ParentSide);
+  void MaybeSendSyncMessage(bool aLockHeld) {
+    Maybe<MonitorAutoLock> lock;
+    if (!aLockHeld) {
+      lock.emplace(*gMonitor);
+    }
+
+    if (!mSyncMessage) {
+      return;
+    }
+
+    PrintSpew("ForwardSyncMsg %s\n",
+              IPC::StringFromIPCMessageType(mSyncMessage->type()));
+
+    MOZ_RELEASE_ASSERT(!mSyncMessageReply);
+    mSyncMessageReply = new Message();
+    if (mSyncMessageIsCall
+        ? !mOpposite->GetIPCChannel()->Call(mSyncMessage, mSyncMessageReply)
+        : !mOpposite->GetIPCChannel()->Send(mSyncMessage, mSyncMessageReply)) {
+      MOZ_RELEASE_ASSERT(mSide == ipc::ChildSide);
       BeginShutdown();
     }
 
-    MonitorAutoLock lock(*gMonitor);
-    *aReply = nReply;
-    gMonitor->Notify();
+    mSyncMessage = nullptr;
+
+    gMonitor->NotifyAll();
+  }
+
+  static void StaticMaybeSendSyncMessage(MiddlemanProtocol* aProtocol) {
+    aProtocol->MaybeSendSyncMessage(false);
+  }
+
+  void HandleSyncMessage(const Message& aMessage, Message*& aReply, bool aCall) {
+    MOZ_RELEASE_ASSERT(mOppositeMessageLoop);
+
+    mSyncMessage = new Message();
+    mSyncMessage->CopyFrom(aMessage);
+    mSyncMessageIsCall = aCall;
+
+    mOppositeMessageLoop->PostTask(
+        NewRunnableFunction("StaticMaybeSendSyncMessage", StaticMaybeSendSyncMessage, this));
+
+    if (mSide == ipc::ChildSide) {
+      AutoMarkMainThreadWaitingForIPDLReply blocked;
+      while (!mSyncMessageReply) {
+        MOZ_CRASH("NYI");
+      }
+    } else {
+      MonitorAutoLock lock(*gMonitor);
+
+      // If the main thread is blocked waiting for the recording child to pause,
+      // wake it up so it can call MaybeHandlePendingSyncMessage().
+      gMonitor->NotifyAll();
+
+      while (!mSyncMessageReply) {
+        gMonitor->Wait();
+      }
+    }
+
+    aReply = mSyncMessageReply;
+    mSyncMessageReply = nullptr;
+
+    PrintSpew("SyncMsgDone\n");
   }
 
   virtual Result OnMessageReceived(const Message& aMessage,
                                    Message*& aReply) override {
-    MOZ_RELEASE_ASSERT(mOppositeMessageLoop);
-
-    Message* nMessage = new Message();
-    nMessage->CopyFrom(aMessage);
-    mOppositeMessageLoop->PostTask(
-        NewRunnableFunction("ForwardMessageSync", ForwardMessageSync, mOpposite,
-                            nMessage, &aReply));
-
-    if (mSide == ipc::ChildSide) {
-      AutoMarkMainThreadWaitingForIPDLReply blocked;
-      while (!aReply) {
-        MOZ_CRASH("NYI");
-      }
-    } else {
-      MonitorAutoLock lock(*gMonitor);
-      while (!aReply) {
-        gMonitor->Wait();
-      }
-    }
-
-    PrintSpew("SyncMsgDone\n");
+    HandleSyncMessage(aMessage, aReply, false);
     return MsgProcessed;
   }
 
-  static void ForwardCallMessage(MiddlemanProtocol* aProtocol,
-                                 Message* aMessage, Message** aReply) {
-    PrintSpew("ForwardSyncCall %s\n",
-              IPC::StringFromIPCMessageType(aMessage->type()));
-
-    MOZ_RELEASE_ASSERT(!*aReply);
-    Message* nReply = new Message();
-    if (!aProtocol->GetIPCChannel()->Call(aMessage, nReply)) {
-      MOZ_RELEASE_ASSERT(aProtocol->mSide == ipc::ParentSide);
-      BeginShutdown();
-    }
-
-    MonitorAutoLock lock(*gMonitor);
-    *aReply = nReply;
-    gMonitor->Notify();
-  }
-
   virtual Result OnCallReceived(const Message& aMessage,
                                 Message*& aReply) override {
-    MOZ_RELEASE_ASSERT(mOppositeMessageLoop);
-
-    Message* nMessage = new Message();
-    nMessage->CopyFrom(aMessage);
-    mOppositeMessageLoop->PostTask(
-        NewRunnableFunction("ForwardCallMessage", ForwardCallMessage, mOpposite,
-                            nMessage, &aReply));
-
-    if (mSide == ipc::ChildSide) {
-      AutoMarkMainThreadWaitingForIPDLReply blocked;
-      while (!aReply) {
-        MOZ_CRASH("NYI");
-      }
-    } else {
-      MonitorAutoLock lock(*gMonitor);
-      while (!aReply) {
-        gMonitor->Wait();
-      }
-    }
-
-    PrintSpew("SyncCallDone\n");
+    HandleSyncMessage(aMessage, aReply, true);
     return MsgProcessed;
   }
 
@@ -357,6 +351,12 @@ class MiddlemanProtocol : public ipc::IToplevelProtocol {
 static MiddlemanProtocol* gChildProtocol;
 static MiddlemanProtocol* gParentProtocol;
 
+void MaybeHandlePendingSyncMessage() {
+  if (gParentProtocol) {
+    gParentProtocol->MaybeSendSyncMessage(true);
+  }
+}
+
 ipc::MessageChannel* ChannelToUIProcess() {
   return gChildProtocol->GetIPCChannel();
 }
diff --git a/toolkit/recordreplay/ipc/ParentInternal.h b/toolkit/recordreplay/ipc/ParentInternal.h
index 943a324c21e7..c4b7c2f2baac 100644
--- a/toolkit/recordreplay/ipc/ParentInternal.h
+++ b/toolkit/recordreplay/ipc/ParentInternal.h
@@ -51,6 +51,11 @@ bool MainThreadIsWaitingForIPDLReply();
 // to block while waiting on an IPDL reply from the child.
 void ResumeBeforeWaitingForIPDLReply();
 
+// Immediately forward any sync child->parent IPDL message. These are sent on
+// the main thread, which might be blocked waiting for a response from the
+// recording child and unable to run an event loop.
+void MaybeHandlePendingSyncMessage();
+
 // Initialize state which handles incoming IPDL messages from the UI and
 // recording child processes.
 void InitializeForwarding();