Emilio Pozuelo of Collabora has shared that he's been working on helping Mozilla with the GTK+ 3 porting of Firefox in an official capacity at the independent, open-source consulting firm. Besides getting the GTK3 user-interface working, he's also started to get Firefox running on Wayland. Overall, the GTK3 work appears to be going well for Firefox though some will argue that it's long overdue to abandon the GTK2 interface.
While GTK+ 3.x is Wayland-compatible, there's various X11-specific code within Firefox that he's had to block in order to get the web-browser running on Wayland.
Those wishing to learn more about this work can read Emilio's blog post on the GTK3/Wayland Firefox topic and via this Mozilla bug report that's tracking the Firefox support for Wayland without any X dependencies.
Meanwhile, Google's Chromium browser has been running great on Wayland thanks to efforts by Intel's Open-Source Technology center.