Using XPather with XML documents in Firefox
Firefox displays XML files without stylesheets in its own default style sheet (instead of displaying the XML itself). This is nice when you want a human-readable representation of the XML file but it falls flat when you want to use XPather to examine the actual XML (instead you get the DOM structure for the transformed HTML file).
If you want to see the actual XML source in XPather, you need to turn off the default XML stylesheet: with the Web Developer extension installed, select Tools → Web Developer → CSS → Disable Styles → Browser Default Styles.
Once you do that, you can use XPather to examine the actual XML source.