Pretty interesting article on how Firefox has been evolving over the last 15-ish years.
I remember when Wrox Press asked me if I knew of anyone that knew about XUL, because their "hype wagon" thought that XUL was the next big thing, and they wanted a book on that. And it was, though after a fashion much smaller than I expect Wrox was imagining.
XUL and XPCOM (which is kinda like COM, which was Microsoft's fork of DCE's DCOM, which was an extendible API system kinda like CORBA...) are now being actively deprecated. A pain in the neck to anyone that had devoted a lot of resources to XUL, but probably not something others care about terribly much.