Yes, the UX is raw for now; hopefully Mozilla will make it simpler to enable after a few versions of testing and confirming it doesn't cause crashes for half the linux userbase. It makes sense to do little early advertising of such a feature, and to initially hide it behind flags & env. vars only within reach of "power users", because these fresh-out-of-the-oven bits are a complex assembly with heavy crash potential, depending on your hardware and drivers up-to-date-ness. As the implementer, you want your initial users to know what they are doing, and you want to maximize the chances they're technically proficient enough to gather the mountain of gpu/driver/environment debug info you'll need for a bug report to be actionable.

In the meantime, to save you rummaging around bugzilla, follow https://wiki.archlinux.org/index.php/Firefox#Hardware_video_... , it's the most constantly {up-to-date, concise, reliable} source you'll find. You don't have to be using Arch, the information contained in this section applies to any Linux/Firefox distro.

Note that Firefox isn't the only browser to struggle with it; see https://wiki.archlinux.org/index.php/Chromium#Hardware_video... for the similarly non-obvious instructions in Chromium (and by the way, currently it's plain impossible in Chrome).

Props to Martin Stransky from RedHat who implemented this for both Wayland and Xorg during the last months.