Desktop Linux is in its never-ending process of replacing old displaying system with new one. The process is long and not really transparent, because the two displaying systems were designed in completely different times for different hardware and with different security concerns in mind, therefore the X11 clients (all the software that was ever made or ported to Linux) are very much incompatible with Wayland. For backwards compatibility there’s Xwayland, which provides full blown Xorg server running on top of Wayland compositor with all the things X11 app requires. Until now, Firefox, even though had its Wayland backend as WIP feature (possible to activate with environment variable MOZ_ENABLE_WAYLAND=1) it defaulted to Xwayland on Wayland sessions. It now uses native Wayland backend by default providing better efficiency, DPI scaling, touchpad gestures etc