it is currently possible to load a mismatching XUL file for the running DF. This is not a real problem, but obviously XUL files will usually be specific to a certain DF.
It would be more intuitive for the UA to inform the user if this is the case by showing a GUI dialog.
It would probably make sense to add a tag to the XUL file that contains the checksum or md5 hash of the DF file that it has been created for.
This would make it possible to ensure that the correct XUL is getting loaded and executed.
When a mismatch is detected, the user could get a notice saying "This XUL file has apparently been written for a different DF (or not yet updated), load anyway?"
Another idea might be to allow the user updating the checksum/hash in the XUL file, so that it matches the currently running loaded DF.