title: Shortcut does not work for firefox addon
body: <p>I've assigned a shortcut (ctrl+shift+z) for my firefox addon menu item by following code in xul file
  but unfortunately it does not work. what is the problem?</p>
  
  <pre><code>  &lt;keyset id="mainKeyset"&gt;
      &lt;key id="key_convert" key="z" modifiers="accel shift"/&gt;
    &lt;/keyset&gt;
  
    &lt;menupopup id="menu_ToolsPopup"&gt; 
  
          &lt;menuitem id="myextension-toolsmenuitem" 
              label="My Extension" 
        key="key_convert" 
              insertbefore="sanitizeSeparator"
      oncommand="myextension.test()" /&gt;
  
  &lt;/menupopup&gt;
  </code></pre>
  
  <p></p>