diff --git a/browser/components/sessionstore/test/browser_privatetabs.js b/browser/components/sessionstore/test/browser_privatetabs.js
index 625046478758..cc02e56cfa71 100644
--- a/browser/components/sessionstore/test/browser_privatetabs.js
+++ b/browser/components/sessionstore/test/browser_privatetabs.js
@@ -15,13 +15,15 @@ add_task(function() {
   try {
     // Setup a public tab and a private tab
     info("Setting up public tab");
-    tab1 = yield BrowserTestUtils.openNewForegroundTab(gBrowser, URL_PUBLIC);
+    tab1 = gBrowser.addTab(URL_PUBLIC);
+    yield promiseBrowserLoaded(tab1.linkedBrowser);
 
     info("Setting up private tab");
-    tab2 = yield BrowserTestUtils.openNewForegroundTab(gBrowser);
+    tab2 = gBrowser.addTab();
+    yield promiseBrowserLoaded(tab2.linkedBrowser);
     yield setUsePrivateBrowsing(tab2.linkedBrowser, true);
     tab2.linkedBrowser.loadURI(URL_PRIVATE);
-    yield BrowserTestUtils.browserLoaded(tab2.linkedBrowser, false, URL_PRIVATE);
+    yield promiseBrowserLoaded(tab2.linkedBrowser);
 
     info("Flush to make sure chrome received all data.");
     yield TabStateFlusher.flush(tab1.linkedBrowser);
@@ -30,15 +32,16 @@ add_task(function() {
     info("Checking out state");
     let state = yield promiseRecoveryFileContents();
 
+    info("State: " + state);
     // Ensure that sessionstore.js only knows about the public tab
     ok(state.indexOf(URL_PUBLIC) != -1, "State contains public tab");
     ok(state.indexOf(URL_PRIVATE) == -1, "State does not contain private tab");
 
     // Ensure that we can close and undo close the public tab but not the private tab
-    yield BrowserTestUtils.removeTab(tab2);
+    gBrowser.removeTab(tab2);
     tab2 = null;
 
-    yield BrowserTestUtils.removeTab(tab1);
+    gBrowser.removeTab(tab1);
     tab1 = null;
 
     tab1 = ss.undoCloseTab(window, 0);
@@ -48,10 +51,10 @@ add_task(function() {
 
   } finally {
     if (tab1) {
-      yield BrowserTestUtils.removeTab(tab1);
+      gBrowser.removeTab(tab1);
     }
     if (tab2) {
-      yield BrowserTestUtils.removeTab(tab2);
+      gBrowser.removeTab(tab2);
     }
   }
 });
@@ -64,13 +67,14 @@ add_task(function () {
   forgetClosedWindows();
 
   // Create a new window to attach our frame script to.
-  let win = yield BrowserTestUtils.openNewBrowserWindow();
+  let win = yield promiseNewWindowLoaded();
   let mm = win.getGroupMessageManager("browsers");
   mm.loadFrameScript(FRAME_SCRIPT, true);
 
   // Create a new tab in the new window that will load the frame script.
-  let tab = yield BrowserTestUtils.openNewForegroundTab(win.gBrowser, "about:mozilla");
+  let tab = win.gBrowser.addTab("about:mozilla");
   let browser = tab.linkedBrowser;
+  yield promiseBrowserLoaded(browser);
   yield TabStateFlusher.flush(browser);
 
   // Check that we consider the tab as private.
@@ -78,12 +82,13 @@ add_task(function () {
   ok(state.isPrivate, "tab considered private");
 
   // Ensure we don't allow restoring closed private tabs in non-private windows.
-  yield BrowserTestUtils.removeTab(tab);
+  win.gBrowser.removeTab(tab);
   is(ss.getClosedTabCount(win), 0, "no tabs to restore");
 
   // Create a new tab in the new window that will load the frame script.
-  tab = yield BrowserTestUtils.openNewForegroundTab(win.gBrowser, "about:mozilla");
+  tab = win.gBrowser.addTab("about:mozilla");
   browser = tab.linkedBrowser;
+  yield promiseBrowserLoaded(browser);
   yield TabStateFlusher.flush(browser);
 
   // Check that we consider the tab as private.
@@ -101,11 +106,12 @@ add_task(function () {
   forgetClosedWindows();
 
   // Create a new window to attach our frame script to.
-  let win = yield BrowserTestUtils.openNewBrowserWindow({private: true});
+  let win = yield promiseNewWindowLoaded({private: true});
 
   // Create a new tab in the new window that will load the frame script.
-  let tab = yield BrowserTestUtils.openNewForegroundTab(win.gBrowser, "about:mozilla");
+  let tab = win.gBrowser.addTab("about:mozilla");
   let browser = tab.linkedBrowser;
+  yield promiseBrowserLoaded(browser);
   yield TabStateFlusher.flush(browser);
 
   // Check that we consider the tab as private.
@@ -113,7 +119,7 @@ add_task(function () {
   ok(state.isPrivate, "tab considered private");
 
   // Ensure that closed tabs in a private windows can be restored.
-  yield BrowserTestUtils.removeTab(tab);
+  win.gBrowser.removeTab(tab);
   is(ss.getClosedTabCount(win), 1, "there is a single tab to restore");
 
   // Ensure that closed private windows can never be restored.
diff --git a/browser/components/sessionstore/test/head.js b/browser/components/sessionstore/test/head.js
index 5ed7160c2370..565baaeb7461 100644
--- a/browser/components/sessionstore/test/head.js
+++ b/browser/components/sessionstore/test/head.js
@@ -23,10 +23,6 @@ registerCleanupFunction(() => {
   for (let script of FRAME_SCRIPTS) {
     mm.removeDelayedFrameScript(script, true);
   }
-
-  // Force a garbage collect after the end of each test run, to make sure that it
-  // won't interfere with the timing of the next test to be run from the suite.
-  window.QueryInterface(Ci.nsIInterfaceRequestor).getInterface(Ci.nsIDOMWindowUtils).garbageCollect();
 });
 
 const {Promise} = Cu.import("resource://gre/modules/Promise.jsm", {});
diff --git a/toolkit/modules/PrivateBrowsingUtils.jsm b/toolkit/modules/PrivateBrowsingUtils.jsm
index 6eff48336fb7..6a84eef93fa8 100644
--- a/toolkit/modules/PrivateBrowsingUtils.jsm
+++ b/toolkit/modules/PrivateBrowsingUtils.jsm
@@ -4,9 +4,7 @@
 
 this.EXPORTED_SYMBOLS = ["PrivateBrowsingUtils"];
 
-const {classes: Cc, interfaces: Ci, utils: Cu} = Components;
-
-Cu.import("resource://gre/modules/Services.jsm");
+Components.utils.import("resource://gre/modules/Services.jsm");
 
 const kAutoStartPref = "browser.privatebrowsing.autostart";
 
@@ -14,11 +12,14 @@ const kAutoStartPref = "browser.privatebrowsing.autostart";
 // line for the current session.
 var gTemporaryAutoStartMode = false;
 
+const Cc = Components.classes;
+const Ci = Components.interfaces;
+
 this.PrivateBrowsingUtils = {
   // Rather than passing content windows to this function, please use
   // isBrowserPrivate since it works with e10s.
   isWindowPrivate: function pbu_isWindowPrivate(aWindow) {
-    if (!(aWindow instanceof Ci.nsIDOMChromeWindow)) {
+    if (!(aWindow instanceof Components.interfaces.nsIDOMChromeWindow)) {
       dump("WARNING: content window passed to PrivateBrowsingUtils.isWindowPrivate. " +
            "Use isContentWindowPrivate instead (but only for frame scripts).\n"
            + new Error().stack);
@@ -44,15 +45,9 @@ this.PrivateBrowsingUtils = {
   },
 
   privacyContextFromWindow: function pbu_privacyContextFromWindow(aWindow) {
-    let context = { usePrivateBrowsing: false };
-    try {
-      context = aWindow.QueryInterface(Ci.nsIInterfaceRequestor)
-                       .getInterface(Ci.nsIWebNavigation)
-                       .QueryInterface(Ci.nsILoadContext);
-    } catch (ex) {
-      Cu.reportError(ex);
-    }
-    return context;
+    return aWindow.QueryInterface(Ci.nsIInterfaceRequestor)
+                  .getInterface(Ci.nsIWebNavigation)
+                  .QueryInterface(Ci.nsILoadContext);
   },
 
   addToTrackingAllowlist(aURI) {
@@ -86,7 +81,7 @@ this.PrivateBrowsingUtils = {
   },
 
   whenHiddenPrivateWindowReady: function pbu_whenHiddenPrivateWindowReady(cb) {
-    Cu.import("resource://gre/modules/Timer.jsm");
+    Components.utils.import("resource://gre/modules/Timer.jsm");
 
     let win = Services.appShell.hiddenPrivateDOMWindow;
     function isNotLoaded() {