Fixed some sloppy programming causing null access keys to show up in the menu in Firefox 20

lib/ui.js

 /*
  * This file is part of the Adblock Plus,
  * Copyright (C) 2006-2012 Eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
  * Adblock Plus is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 1623 matching lines @@
     {
       let item = items[i];
       let match = /^abp-key-/.exec(item.getAttribute("key"));
       if (!match)
         continue;
 
       let name = match.input.substr(match.index + match[0].length);
       if (!this.hotkeys)
         this.configureKeys(window);
       if (name in this.hotkeys)
-        item.setAttribute("acceltext", KeySelector.getTextForKey(this.hotkeys[name]));
+      {
+        let text = KeySelector.getTextForKey(this.hotkeys[name]);
+        if (text)
+          item.setAttribute("acceltext", text);
+        else
+          item.removeAttribute("acceltext");
+      }
     }
 
     hideElement(prefix + "contributebutton", Prefs.hideContributeButton);
   },
 
   /**
    * Adds Adblock Plus menu items to the content area context menu when it shows
    * up.
    */
   fillContentContextMenu: function(/**Element*/ popup)
@@ skipping 267 matching lines @@
   ["abp-command-contribute", "command", UI.openContributePage.bind(UI)],
   ["abp-command-contribute-hide", "command", UI.hideContributeButton.bind(UI)]
 ];
 
 onShutdown.add(function()
 {
   for (let window in UI.applicationWindows)
     if (UI.isBottombarOpen(window))
       UI.toggleBottombar(window);
 });