diff -r 000000000000 -r 6474c204b198 browser/components/customizableui/test/browser_889120_customize_tab_merging.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/browser/components/customizableui/test/browser_889120_customize_tab_merging.js	Wed Dec 31 06:09:35 2014 +0100
@@ -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/. */
+
+"use strict";
+
+const kTestToolbarId = "test-empty-drag";
+
+// Attempting to switch quickly from one tab to another to see whether the state changes
+// correctly.
+add_task(function CheckBasicCustomizeMode() {
+  yield startCustomizing();
+  ok(CustomizationHandler.isCustomizing(), "We should be in customize mode");
+  yield endCustomizing();
+  ok(!CustomizationHandler.isCustomizing(), "We should not be in customize mode");
+});
+add_task(function CheckQuickCustomizeModeSwitch() {
+  let tab1 = gBrowser.addTab("about:newtab");
+  gBrowser.selectedTab = tab1;
+  let tab2 = gBrowser.addTab("about:customizing");
+  let tab3 = gBrowser.addTab("about:newtab");
+  gBrowser.selectedTab = tab2;
+  try {
+    yield waitForCondition(() => CustomizationHandler.isEnteringCustomizeMode);
+  } catch (ex) {
+    Cu.reportError(ex);
+  }
+  ok(CustomizationHandler.isEnteringCustomizeMode, "Should be entering customize mode");
+  gBrowser.selectedTab = tab3;
+  try {
+    yield waitForCondition(() => !CustomizationHandler.isEnteringCustomizeMode && !CustomizationHandler.isCustomizing());
+  } catch (ex) {
+    Cu.reportError(ex);
+  }
+  ok(!CustomizationHandler.isCustomizing(), "Should not be entering customize mode");
+  gBrowser.removeTab(tab1);
+  gBrowser.removeTab(tab2);
+  gBrowser.removeTab(tab3);
+});
+
+add_task(function asyncCleanup() {
+  yield endCustomizing();
+});
+