<?xml version="1.0" encoding="UTF-8"?>
<?xml-stylesheet href="chrome://global/skin/global.css" type="text/css"?>
<window id="MyWindow"
title=""
xmlns="
http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
width="800"
height="600"
onload="revealTab()">
<script type="application/x-javascript">
function revealTab(){
document.getElementById("fourthTab").setAttribute("hidden", "false");
}
</script>
<tabbox>
<tabs>
<tab label="first"/>
<tab label="second"/>
<tab label="third"/>
<tab id="fourthTab" label="fourth" hidden="true"/>
</tabs>
<tabpanels>
<tabpanel>
<label value="First page"/>
</tabpanel>
<tabpanel>
<label value="second page"/>
</tabpanel>
<tabpanel>
<label value="third page"/>
</tabpanel>
<tabpanel>
<label value="fourth page"/>
</tabpanel>
</tabpanels>
</tabbox>
</window>
