Something

<popupset>
  <popup id="toolbarmenu">
    <menuitem label="&gbrain.menu.toggle;"
     oncommand="gbrain.toggle(0)"/>
    <menuitem label="&gbrain.menu.visit;"
     oncommand="gbrain.openInCurrentWindow('http://www.google.com/bookmarks/lookup?q=label:!gbrain');"/>
    <menuitem label="&gbrain.menu.addex;"
     oncommand="gbrain.addException()"/>
    <menuitem label="&gbrain.menu.config;"
     oncommand="gbrain.openPrefs();"/>
  </popup>
  </popupset>


  <toolbarpalette id="BrowserToolbarPalette">
  <toolbarbutton id="gbrainbutton" class="toolbarbutton-1 gbrain-enabled"
    label="&gbrainbutton.label;" tooltiptext="&gbrainbutton.label;"
    onclick="gbrain.onToolbarButtonClick(event);" context="toolbarmenu"/>
  </toolbarpalette>