diff -r 000000000000 -r 6474c204b198 browser/components/customizableui/test/browser_877447_skip_missing_ids.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/browser/components/customizableui/test/browser_877447_skip_missing_ids.js	Wed Dec 31 06:09:35 2014 +0100
@@ -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/. */
+
+"use strict";
+
+registerCleanupFunction(removeCustomToolbars);
+
+add_task(function skipMissingIDS() {
+  const kButtonId = "look-at-me-disappear-button";
+  CustomizableUI.reset();
+  ok(CustomizableUI.inDefaultState, "Should be in the default state.");
+  let btn = createDummyXULButton(kButtonId, "Gone!");
+  CustomizableUI.addWidgetToArea(kButtonId, CustomizableUI.AREA_NAVBAR);
+  ok(!CustomizableUI.inDefaultState, "Should no longer be in the default state.");
+  is(btn.parentNode.parentNode.id, CustomizableUI.AREA_NAVBAR, "Button should be in navbar");
+  btn.remove();
+  is(btn.parentNode, null, "Button is no longer in the navbar");
+  ok(CustomizableUI.inDefaultState, "Should be back in the default state, " +
+                                    "despite unknown button ID in placements.");
+});
+
+add_task(function asyncCleanup() {
+  yield resetCustomization();
+});