As of last nightly (20181115100051), Firefox now supports Wayland on Linux, thanks to the work from Martin Stransky and Jan Horak, mostly.
Before that, it was possible to build your own Firefox with Wayland support (and Fedora does it), but now the downloads from mozilla.org come with Wayland support out of the box for the first time.
However, being experimental and all, the Wayland support is not enabled by default, meaning by default, you’ll still be using XWayland. To enable wayland support, first set the GDK_BACKEND environment variable to wayland.
To verify whether Wayland support is enabled, go to about:support, and check “WebGL 1 Driver WSI Info” and/or “WebGL 2 Driver WSI Info”. If they say something about GLX, Wayland support is not enabled. If they say something about EGL, it is. I filed a bug to make it more obvious what is being used.
It’s probably still a long way before Firefox enables Wayland support on Wayland by default, but we reached a major milestone here. Please test and report any bug you encounter.
Update: I should mention that should you build your own Firefox, as long as your Gtk+ headers come with Wayland support, you’ll end up with the same Wayland support as the one shipped by Mozilla.