My local cairo-gtk2 build of Firefox 49 reliably crashes anytime you try to use the GTK file dialog. Does it work for you (arch or gentoo)?
Not only I haven't had any issues with it so far, it even looks better than Arch's official package (that's built with gtk3).
The issues have been present with gtk >= 3.20, and they should be fixed with release 50 and 51: