I know you're being facetious, but Mozilla shipped Motif builds for a really long time after most of the world had switched to GTK. Far longer than seemed reasonable at the time. Given that the majority of GTK-running systems support GTK2, and that the GTK3 builds of Firefox require GTK2 already, it seems very strange that the default distribution would bother with GTK3 at this time. I'm sure they have good reasons; I'm just curious about what they are.