Building Firefox with GTK3 works quite well nowadays. I've been using it for months now without any problems. Among the big advantages are things like smooth scrolling and better visual integration with gtk3 environments. It is still considered somewhat experimental however and knowing Mozilla, will not be officially used for quite some while. May I suggest a (masked?) useflag to build with gtk3? All that would be needed is to replace in .mozconfig: --enable-default-toolkit=cairo-gtk2 with --enable-default-toolkit=cairo-gtk3
GTK3 builds are very very bugs. Well they do offer some benefits they do cause more headaches then they are worth. The mozilla herd is monitoring the upstream bug on it and will add gtk3 support once most of the bugs are ironed out.
You can add "use gtk3 && mozconfig-annotate '' --enable-default-toolkit=cairo-gtk3" to the src_configure of your ebuild. However, gtk3 flag requires system-cairo to be enabled too.
