As regard doing anything to Firefox (if it were to be done) it was my understanding that the entire Firefox interface is written in XUL. That includes everything from menus to dialogue boxes and toolbars. Surely given this, and adjustments made to GTK+ would have no affect what-so-ever on the interface of Firefox, except *possible* to change it's apperance (i.e. colour, widgets, etc.).
It's easy to see this in practice: all skins are written in XUL and you can change literally what *anything* in the interface looks like using CSS. The interface is actually rendered by the Gecko engine just as the web pages it contains are. At least, this is what I had gathered (and I haven't seen any evidence to prove otherwise).
Anyway, the whole of Firefox is skinned. It's appeaance can be changed to RISC OS like just by changing the skin mostly. Other functionalities would still need changing however (such as how a three button mouse is dealt with differently to on Windows).