When trying to parse XML you get:
Error: Permission denied to get property Element.childNodes
Source file: http://www.w3schools.com/XML/tryit_view.asp
Line: 24
Apparently it is an issue with FF itself.
https://bugzilla.mozilla.org/show_bug.cgi?id=434522
https://bugzilla.mozilla.org/show_bug.cgi?id=385676
Hopefully, since both issues are recently marked as solved, this issue would be resolved in the next FF update.
Update:
This issue was solved in FF 3.0.3
Update 27 Jan. 2009:
This is finally solved with Firebug 1.3
