<?xml version="1.0" encoding="UTF-8"?>
<?xml-stylesheet href="chrome://global/skin/" 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>