<window xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
<script id="script" xmlns="http://www.w3.org/1999/xhtml">
function doe(){
document.getElementById('a').removeAttribute('style');
}
setTimeout(doe,100);
</script>
<box id="a" style="position: absolute;">
  <menuitem sizetopopup="always">
    <menupopup style="position: absolute;"/>
  </menuitem>

  <box style="position: fixed;">
    <tree>
      <treecol>
        <treecol/>
      </treecol>
    </tree>
  </box>
</box>
</window>