diff -r 000000000000 -r 6474c204b198 browser/components/customizableui/test/browser_981305_separator_insertion.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/browser/components/customizableui/test/browser_981305_separator_insertion.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,73 @@
+/* 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";
+
+let tempElements = [];
+
+function insertTempItemsIntoMenu(parentMenu) {
+  // Last element is null to insert at the end:
+  let beforeEls = [parentMenu.firstChild, parentMenu.lastChild, null];
+  for (let i = 0; i < beforeEls.length; i++) {
+    let sep = document.createElement("menuseparator");
+    tempElements.push(sep);
+    parentMenu.insertBefore(sep, beforeEls[i]);
+    let menu = document.createElement("menu");
+    tempElements.push(menu);
+    parentMenu.insertBefore(menu, beforeEls[i]);
+    // And another separator for good measure:
+    sep = document.createElement("menuseparator");
+    tempElements.push(sep);
+    parentMenu.insertBefore(sep, beforeEls[i]);
+  }
+}
+
+function checkSeparatorInsertion(menuId, buttonId, subviewId) {
+  return function() {
+    info("Checking for duplicate separators in " + buttonId + " widget");
+    let menu = document.getElementById(menuId);
+    insertTempItemsIntoMenu(menu);
+
+    let placement = CustomizableUI.getPlacementOfWidget(buttonId);
+    let changedPlacement = false;
+    if (!placement || placement.area != CustomizableUI.AREA_PANEL) {
+      CustomizableUI.addWidgetToArea(buttonId, CustomizableUI.AREA_PANEL);
+      changedPlacement = true;
+    }
+    yield PanelUI.show();
+
+    let button = document.getElementById(buttonId);
+    button.click();
+
+    yield waitForCondition(() => !PanelUI.multiView.hasAttribute("transitioning"));
+    let subview = document.getElementById(subviewId);
+    ok(subview.firstChild, "Subview should have a kid");
+    is(subview.firstChild.localName, "toolbarbutton", "There should be no separators to start with");
+
+    for (let kid of subview.children) {
+      if (kid.localName == "menuseparator") {
+        ok(kid.previousSibling && kid.previousSibling.localName != "menuseparator",
+           "Separators should never have another separator next to them, and should never be the first node.");
+      }
+    }
+
+    let panelHiddenPromise = promisePanelHidden(window);
+    PanelUI.hide();
+    yield panelHiddenPromise;
+
+    if (changedPlacement) {
+      CustomizableUI.reset();
+    }
+  };
+}
+
+add_task(checkSeparatorInsertion("menuWebDeveloperPopup", "developer-button", "PanelUI-developerItems"));
+add_task(checkSeparatorInsertion("viewSidebarMenu", "sidebar-button", "PanelUI-sidebarItems"));
+
+registerCleanupFunction(function() {
+  for (let el of tempElements) {
+    el.remove();
+  }
+  tempElements = null;
+});