read and writing file using mozilla xul - xul

Question

I've got a problem with reading and writing files in mozilla xul.
At first I want simply to read path to file(to check whether I/O works)
So I wrote this code
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<window xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul" id="window" title="title">
<script>
Components.utils.import("resource://gre/modules/FileUtils.jsm");
var file = FileUtils.getFile("Desk", ["temp.xml"]);
alert(file.path);
</script>
</window>
It should show alert window with path to temp.xml(this file exists on desktop). But it shows nothing in mozilla firefox.
what's the problem?

Answer 1

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>