Fennec used to work without GTK? And Firefox on Android does, but there it's probably using another toolkit. It causes this exact problem - porting Firefox to a new system is impossible without rewriting all that.
Yeah, Android Firefox uses the native Android UI toolkit. A long time ago, they had it in XUL, which allowed for some really nice customization and just in general a more conform behaviour with the desktop browser, but performance back then was really just terrible, so that's why they changed it to the native Android UI toolkit.
