title: Firefox extension error - value does not implement interface WindowProxy interface
body: <pre><code>var mouse = document.createEvent('MouseEvent');
  mouse.initMouseEvent("mousedown",true,false,window,1, 0, 0, 0, 0, false, false, false, false, 0, null);
  </code></pre>
  
  <p>The above code is present in my content script. But when this code is invoked, firefox throws following error. <code>An exception occurred. TypeError: Value does not implement interface WindowProxy</code></p>
  
  <p>I think I am passing invalid <code>window</code> object to <code>initMouseEvent</code>. But I don't see a way to fix it.</p>