I can't find the link, but I do remember a technical article on netscape.com that said that Mozilla is using XML to define GUI components... the XML is apparently then parsed into appropriate C++ code. I will admit that this subject (defining XML-based GUI) mildly interests me. I might try my hand at something similar, myself.
Disclaimer: I am not a mozilla developer, so this story is, at best, hearsay, until I find the link. ;)
I've seen similar stuff with Glade, where the GUI details are stored in XML, until the actual code is generated. Since Glade also does Perl (check CPAN), it can be done :)
Greetz
Beatnik
... Quidquid perl dictum sit, altum viditur.