--- a/testing/mochitest/browser-test.js
+++ b/testing/mochitest/browser-test.js
@@ -506,20 +506,35 @@ Tester.prototype = {
     this._globalProperties = Object.keys(window);
     this._globalPropertyWhitelist = [
       "navigator", "constructor", "top",
       "Application",
       "__SS_tabsToRestore", "__SSi",
       "webConsoleCommandController",
     ];
 
-    if (this.tests.length)
-      this.waitForGraphicsTestWindowToBeGone(this.nextTest.bind(this));
-    else
+    if (this.tests.length) {
+      this.waitForWindowsReady().then(() => {
+        this.nextTest();
+      });
+    } else {
       this.finish();
+    }
+  },
+
+  async waitForWindowsReady() {
+    await new Promise(resolve => this.waitForGraphicsTestWindowToBeGone(resolve));
+    await this.promiseMainWindowReady();
+  },
+
+  async promiseMainWindowReady() {
+    if (!gBrowserInit.idleTasksFinished) {
+      await this.TestUtils.topicObserved("browser-idle-startup-tasks-finished",
+                                         subject => subject === window);
+    }
   },
 
   waitForGraphicsTestWindowToBeGone(aCallback) {
     let windowsEnum = Services.wm.getEnumerator(null);
     while (windowsEnum.hasMoreElements()) {
       let win = windowsEnum.getNext();
       if (win != window && !win.closed &&
           win.document.documentURI == "chrome://gfxsanity/content/sanityparent.html") {