For reference, WWW::Mechanize::Firefox uses document.evaluate() to execute an XPath query. I don't know how it relates to ->findNodes and ->find, but the return value will always be DOM nodes.

Javascript in MozRepl::RemoteObject:

...
var xres = doc.evaluate(q,ref,null,XPathResult.ORDERED_NODE_SNAPSHOT_T
+YPE, null );
...
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