<?xml version="1.0"?> <?xml-stylesheet href="chrome://global/skin/" type="text/css"?> <window id="yourwindow" xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"> <script type="text/javascript"> <![CDATA[ function onload() { document.addEventListener("DOMAttrModified", function() {}, true); document.getElementById("test").setAttribute("value", "50"); } ]]> </script> <progressmeter id="test"/> </window>