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.
The transition to Wayland seems to be taking its time, but with how big of an undertaking this is, that only makes sense.
Is Wayland not just another lower level display mechanism for X? I think I need to go read more.