I think it works OOTB if you're on nightly, yeah.

Maybe `GDK_BACKEND=wayland`

It switched a while back, it's MOZ_ENABLE_WAYLAND now (or will be soon?), as far as I understand: https://bugzilla.mozilla.org/show_bug.cgi?id=1522780

Note that perf for Firefox+HiDPI+Wayland-native is noticeably worse than XWayland (regardless of whether it's scaled or Firefox is rendering itself at 2x), and keyboard shortcuts are still not functioning correctly (and apparently only in Sway).