Something

<?xml version="1.0"?>
<!DOCTYPE overlay PUBLIC "-//MOZILLA//DTD XUL V1.0//EN" "http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">

<overlay id="commandextensionOverlay"
         xmlns:html="http://www.w3.org/1999/xhtml"
         xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
         xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">

  <commandset id="allcommands">
    <command id="Tasks:CommandWindow" oncommand="display();" />
  </commandset>

  <menupopup id="popup_tools">
    <menuitem id="menu_commandextension"
              label="Command Extensionx"
              observes="Tasks:CommandWindow"
              class="menu-iconic-wide"/>
  </menupopup>
  
<tabbox id="output_area" >
 <tabs id="output_tabs">
         <tab id="command_input_tab"
                  label="command input"
                  accesskey="w"
          ></tab>
 </tabs>
 
 <tabpanels id="output_tabpanels">
         <tabpanel id="command_input_tabpanel"
                           flex="1"
                           >
                 <hbox id="command_input_hbox" flex="1">
                 <textbox id="command_input_textbox" multiline="true" ></textbox>
                 </hbox>
         </tabpanel>
 </tabpanels>
</tabbox>

<script>
  function display()
  {
    window.alert("display"); 
  }

</script>    
</overlay>