Su Ubuntu 21.04 visto che ormai Wayland é il default ho creato un .desktop per FF in Wayland puro:
cp /usr/share/applications/firefox.desktop ~/.local/share/applications/firefox.desktop
sed 's/Exec=firefox/Exec=env MOZ_ENABLE_WAYLAND=1 firefox/g' -i ~/.local/share/applications/firefox.desktop
about:support
Window Protocol wayland