For most apps there's no difference, but dealing with multi-window apps that can spawn new windows, merge them, display video content in its own window etc. there's a lot of communication that Firefox has to do with the technology that draws its window to the screen.

I guess before now, default Firefox setups would've used XWayland to translate those communications which would've worked fine if not for some overhead and edge cases. This would make Firefox a truly Wayland-native application, when running on Wayland.