diff --git a/browser/base/content/test/performance/browser_appmenu_reflows.js b/browser/base/content/test/performance/browser_appmenu_reflows.js
index 451b9da12d7c..2d26c84c3da8 100644
--- a/browser/base/content/test/performance/browser_appmenu_reflows.js
+++ b/browser/base/content/test/performance/browser_appmenu_reflows.js
@@ -118,7 +118,7 @@ add_task(async function() {
       BrowserTestUtils.waitForEvent(PanelUI.panel, "popuppositioned");
     await PanelUI.show();
     await popupPositioned;
-  }, EXPECTED_APPMENU_OPEN_REFLOWS, window, PanelUI.panel);
+  }, EXPECTED_APPMENU_OPEN_REFLOWS);
 
   // Now open a series of subviews, and then close the appmenu. We
   // should not reflow during any of this.
@@ -160,5 +160,5 @@ add_task(async function() {
     let hidden = BrowserTestUtils.waitForEvent(PanelUI.panel, "popuphidden");
     PanelUI.hide();
     await hidden;
-  }, EXPECTED_APPMENU_SUBVIEW_REFLOWS, window, PanelUI.panel);
+  }, EXPECTED_APPMENU_SUBVIEW_REFLOWS);
 });
diff --git a/browser/base/content/test/performance/browser_tabclose_grow_reflows.js b/browser/base/content/test/performance/browser_tabclose_grow_reflows.js
index e61898af97d2..5ce742719687 100644
--- a/browser/base/content/test/performance/browser_tabclose_grow_reflows.js
+++ b/browser/base/content/test/performance/browser_tabclose_grow_reflows.js
@@ -36,12 +36,6 @@ add_task(async function() {
   const TAB_COUNT_FOR_GROWTH = computeMaxTabCount();
   await createTabs(TAB_COUNT_FOR_GROWTH);
 
-  // Because the tab strip is a scrollable frame, we can't use the
-  // default dirtying function from withReflowObserver and reliably
-  // get reflows for the strip. Instead, we provide a node that's
-  // already in the scrollable frame to dirty - in this case, the
-  // original tab.
-  let origTab = gBrowser.selectedTab;
   let lastTab = gBrowser.tabs[gBrowser.tabs.length - 1];
   await BrowserTestUtils.switchTab(gBrowser, lastTab);
 
@@ -52,7 +46,7 @@ add_task(async function() {
     await BrowserTestUtils.waitForEvent(tab, "transitionend",
       false, e => e.propertyName === "max-width");
     await switchDone;
-  }, EXPECTED_REFLOWS, window, origTab);
+  }, EXPECTED_REFLOWS);
 
   await removeAllButFirstTab();
 });
diff --git a/browser/base/content/test/performance/browser_tabclose_reflows.js b/browser/base/content/test/performance/browser_tabclose_reflows.js
index bc4e1cb2bc08..85089ff533eb 100644
--- a/browser/base/content/test/performance/browser_tabclose_reflows.js
+++ b/browser/base/content/test/performance/browser_tabclose_reflows.js
@@ -22,13 +22,6 @@ const EXPECTED_REFLOWS = [
 add_task(async function() {
   await ensureNoPreloadedBrowser();
 
-  // Because the tab strip is a scrollable frame, we can't use the
-  // default dirtying function from withReflowObserver and reliably
-  // get reflows for the strip. Instead, we provide a node that's
-  // already in the scrollable frame to dirty - in this case, the
-  // original tab.
-  let origTab = gBrowser.selectedTab;
-
   let tab = await BrowserTestUtils.openNewForegroundTab(gBrowser);
   await BrowserTestUtils.waitForCondition(() => tab._fullyOpen);
 
@@ -39,6 +32,6 @@ add_task(async function() {
     await BrowserTestUtils.waitForEvent(tab, "transitionend",
         false, e => e.propertyName === "max-width");
     await switchDone;
-  }, EXPECTED_REFLOWS, window, origTab);
+  }, EXPECTED_REFLOWS);
   is(EXPECTED_REFLOWS.length, 0, "No reflows are expected when closing a tab");
 });
diff --git a/browser/base/content/test/performance/browser_tabopen_reflows.js b/browser/base/content/test/performance/browser_tabopen_reflows.js
index 46c005674748..1fe7363a5e9d 100644
--- a/browser/base/content/test/performance/browser_tabopen_reflows.js
+++ b/browser/base/content/test/performance/browser_tabopen_reflows.js
@@ -27,13 +27,6 @@ const EXPECTED_REFLOWS = [
 add_task(async function() {
   await ensureNoPreloadedBrowser();
 
-  // Because the tab strip is a scrollable frame, we can't use the
-  // default dirtying function from withReflowObserver and reliably
-  // get reflows for the strip. Instead, we provide a node that's
-  // already in the scrollable frame to dirty - in this case, the
-  // original tab.
-  let origTab = gBrowser.selectedTab;
-
   // Add a reflow observer and open a new tab.
   await withReflowObserver(async function() {
     let switchDone = BrowserTestUtils.waitForEvent(window, "TabSwitchDone");
@@ -41,7 +34,7 @@ add_task(async function() {
     await BrowserTestUtils.waitForEvent(gBrowser.selectedTab, "transitionend",
         false, e => e.propertyName === "max-width");
     await switchDone;
-  }, EXPECTED_REFLOWS, window, origTab);
+  }, EXPECTED_REFLOWS);
 
   let switchDone = BrowserTestUtils.waitForEvent(window, "TabSwitchDone");
   await BrowserTestUtils.removeTab(gBrowser.selectedTab);
diff --git a/browser/base/content/test/performance/browser_tabopen_squeeze_reflows.js b/browser/base/content/test/performance/browser_tabopen_squeeze_reflows.js
index a1e555c44edb..6776864c91a1 100644
--- a/browser/base/content/test/performance/browser_tabopen_squeeze_reflows.js
+++ b/browser/base/content/test/performance/browser_tabopen_squeeze_reflows.js
@@ -33,20 +33,13 @@ add_task(async function() {
 
   await createTabs(TAB_COUNT_FOR_SQUEEZE);
 
-  // Because the tab strip is a scrollable frame, we can't use the
-  // default dirtying function from withReflowObserver and reliably
-  // get reflows for the strip. Instead, we provide a node that's
-  // already in the scrollable frame to dirty - in this case, the
-  // original tab.
-  let origTab = gBrowser.selectedTab;
-
   await withReflowObserver(async function() {
     let switchDone = BrowserTestUtils.waitForEvent(window, "TabSwitchDone");
     BrowserOpenTab();
     await BrowserTestUtils.waitForEvent(gBrowser.selectedTab, "transitionend",
       false, e => e.propertyName === "max-width");
     await switchDone;
-  }, EXPECTED_REFLOWS, window, origTab);
+  }, EXPECTED_REFLOWS);
 
   await removeAllButFirstTab();
 });
diff --git a/browser/base/content/test/performance/browser_tabswitch_reflows.js b/browser/base/content/test/performance/browser_tabswitch_reflows.js
index 6d7b0bd2a738..1fe223d34057 100644
--- a/browser/base/content/test/performance/browser_tabswitch_reflows.js
+++ b/browser/base/content/test/performance/browser_tabswitch_reflows.js
@@ -32,13 +32,7 @@ add_task(async function() {
   Assert.equal(EXPECTED_REFLOWS.length, 0,
     "We shouldn't have added any new expected reflows.");
 
-  // Because the tab strip is a scrollable frame, we can't use the
-  // default dirtying function from withReflowObserver and reliably
-  // get reflows for the strip. Instead, we provide a node that's
-  // already in the scrollable frame to dirty - in this case, the
-  // original tab.
   let origTab = gBrowser.selectedTab;
-
   let firstSwitchDone = BrowserTestUtils.waitForEvent(window, "TabSwitchDone");
   let otherTab = await BrowserTestUtils.openNewForegroundTab(gBrowser);
   await firstSwitchDone;
@@ -47,7 +41,7 @@ add_task(async function() {
     let switchDone = BrowserTestUtils.waitForEvent(window, "TabSwitchDone");
     gBrowser.selectedTab = origTab;
     await switchDone;
-  }, EXPECTED_REFLOWS, window, origTab);
+  }, EXPECTED_REFLOWS);
 
   await BrowserTestUtils.removeTab(otherTab);
 });
diff --git a/browser/base/content/test/performance/head.js b/browser/base/content/test/performance/head.js
index 650f5d41df71..32198cb9146a 100644
--- a/browser/base/content/test/performance/head.js
+++ b/browser/base/content/test/performance/head.js
@@ -65,20 +65,17 @@
  *        it defaults to the empty Array, meaning no reflows are expected.
  * @param window (browser window, optional)
  *        The browser window to monitor. Defaults to the current window.
- * @param elemToDirty (DOM node, optional)
- *        Callers can provide a custom DOM node to change some layout style
- *        on in the event that the action being tested is occurring within
- *        a scrollable frame. Otherwise, withReflowObserver defaults to dirtying
- *        the first element child of the window.
  */
-async function withReflowObserver(testFn, expectedStacks = [], win = window, elemToDirty) {
-  if (!elemToDirty) {
-    elemToDirty = win.document.firstElementChild;
-  }
-
-  let i = 0;
-  let dirtyFrameFn = (e) => {
-    elemToDirty.style.margin = (++i % 4) + "px";
+async function withReflowObserver(testFn, expectedStacks = [], win = window) {
+  let dwu = win.QueryInterface(Ci.nsIInterfaceRequestor)
+               .getInterface(Ci.nsIDOMWindowUtils);
+  let dirtyFrameFn = () => {
+    try {
+      dwu.ensureDirtyRootFrame();
+    } catch (e) {
+      // If this fails, we should probably make note of it, but it's not fatal.
+      info("Note: ensureDirtyRootFrame threw an exception.");
+    }
   };
 
   let els = Cc["@mozilla.org/eventlistenerservice;1"]
@@ -120,7 +117,9 @@ async function withReflowObserver(testFn, expectedStacks = [], win = window, ele
     },
 
     reflowInterruptible(start, end) {
-      // We're not interested in interruptible reflows.
+      // We're not interested in interruptible reflows, but might as well take the
+      // opportuntiy to dirty the root frame.
+      dirtyFrameFn();
     },
 
     QueryInterface: XPCOMUtils.generateQI([Ci.nsIReflowObserver,
@@ -148,8 +147,6 @@ async function withReflowObserver(testFn, expectedStacks = [], win = window, ele
 
     els.removeListenerForAllEvents(win, dirtyFrameFn, true);
     docShell.removeWeakReflowObserver(observer);
-
-    elemToDirty.style.margin = "";
   }
 }