You can always add the env var in /etc/profile, though there are likely more fine grained approaches (like editing the firefox.desktop file) that won’t set vars for everything.
Also, if using the flatpak version, you can use flatseal to give it access to that variable under “Environment” or use flatpak override --env=MOZ_ENABLE_WAYLAND=1 org.mozilla.firefox
You can set MOZ_ENABLE_WAYLAND=1 to have it today!
https://wiki.archlinux.org/title/Firefox#Wayland
How do you make it permanent though?
You can always add the env var in /etc/profile, though there are likely more fine grained approaches (like editing the firefox.desktop file) that won’t set vars for everything.
Also, if using the flatpak version, you can use flatseal to give it access to that variable under “Environment” or use
flatpak override --env=MOZ_ENABLE_WAYLAND=1 org.mozilla.firefox