That's called client side window drawing and GTK has it. Needed for Wayland. Maybe good to investigate a little bit before saying things that aren't true.
Not in GTK2 which Firefox uses. It doesn't use GTK3 yet. Qt port of Firefox already exists anyway, but Mozilla didn't find much interest to support it.
Also, I'm not sure how Wayland is going to solve that, since Wayland protocol doesn't mandate for the window manager to use client side decorations. May be you are talking about Weston (which is a reference compositor of the Wayland project). But in practice each major DE will probably create their own compositors which won't necessarily use client side decorations.
