Display xul files in Firefox

[Dave Feustel]

I copied this file (ex-01.html) from the book _Programming Firefox_,
but Firefox displays this file as a blank page. 'view page source'
shows the data as I entered it. What's wrong with the file (or file
name)? 

Thanks.

=====================
<?xml version="1.0"?>
<?xml-stylesheet href="chrome://global/skin/" type="text/css"?>
<window
        id="theWindow"
        title="The Window"
        orient="horizontal"
        width= "400"
        height = "300"
        xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
        xmlns:html="http://www.w3.org/1999/xhtml"
>

<hbox>
  <label value="label 1"/>
   <label value="label 2"/>
    <label value="label 3"/>
</hbox>
<vbox>
  <label value="label 4"/>
   <label value="label 5"/>
    <label value="label 6"/>
</vbox>
</window>