My Firefox shows two problems:
it chokes on alert() with error:
Error: Cannot call openModalWindow on a hidden window = NS_ERROR_NOT_AVAILABLE Source file: resource://gre/components/nsPrompter.js Line: 382
- Your window has no style, so it's "invisible"
Here is a work around with displaying the path in a textbox instead and in browser console
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<?xml-stylesheet href="chrome://global/skin/global.css" type="text/css"?>
<window xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul" id="window" title="title">
<textbox id="text" value="N/A"/>
<script type="application/javascript">
Components.utils.import("resource://gre/modules/FileUtils.jsm");
var file = FileUtils.getFile("Desk", ["temp.xml"]);
document.getElementById("text").value = file.path;
console.log(file.path);
//alert(file.path);
</script>
</window>