According to https://wiki.archlinux.org/index.php/Firefox#Wayland, if you want to see whether you're using the Wayland version of Firefox, go to about:support and look next to "Window Protocol". Wayland can be enabled at runtime using an environment variable: `MOZ_ENABLE_WAYLAND=1 firefox`.

I just updated to Firefox 75 and it's still using x11. If I opt in to using Wayland, it works, but annoyingly I can't detach a tab by dragging it off of the window. So it looks like Wayland support is not quite there yet.