とりあえず

<?xml version="1.0"?>

<?xml-stylesheet href="chrome://global/skin/" type="text/css"?>

<window xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">

  <script>

<![CDATA[

function $(id) {

  return document.getElementById(id);

}
function print(x) {

  var value = $("console").value;

  $("console").value =  x + "\r\n" + value;

}
var i = 0;
function main() {

  print(i++)

}

]]>

  </script>

  <hbox>

    <button label="run" onclick="main()" />

  </hbox>

  <textbox id="console" rows="30" multiline="true" wrap="ture" />

</window>