sample.xul
----------

<?xml version="1.0" encoding="iso-8859-1"?>
<?xml-stylesheet href="style.css" type="text/css"?>
<window xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
        orient="horizontal">

  <vbox>
  <hiperchola>
    <menupopup>
      <menuitem label="avión"/>
      <menuitem label="barco"/>
      <menuitem label="paquetería"/>
    </menupopup>
    </hiperchola>

    <menulist label="barco">
      <menupopup>
        <menuitem label="avión"/>
        <menuitem label="barco"/>
        <menuitem label="paquetería"/>
      </menupopup>
    </menulist>
  </vbox>

</window>


style.css
---------

@import url('chrome://global/skin/');

hiperchola {
  -moz-binding: url('components.xml#hiperchola');
}

components.xml
--------------

<?xml version="1.0" encoding="iso-8859-1"?>
<bindings xmlns="http://www.mozilla.org/xbl"
          xmlns:xul="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
  <binding id="hiperchola">
    <content>
      <xul:label value="Test: "/>
      <xul:menulist>
        <xul:menupopup>
          <children/>
        </xul:menupopup>
      </xul:menulist>
      <children/>
    </content>
  </binding>
</bindings>