Someone

<?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>
function bar(event) {
  event.target.firstChild.textContent = "baz";
return true;
}
</script>

<tooltip id="foo" noautohide="true" onpopupshowing="return bar(event)">
  <label>foo</label>
</tooltip>

<label value="Put your XUL here!" tooltip="foo"/>
</window>