[MaxRegret]

Desktop applications can continue to use the X11 protocol, but then they'll be talking to the Xwayland server, which in turn Wayland to actually draw its clients' windows on the screen.

But it's preferable to have applications directly use the Wayland protocol, which is what Firefox is experimenting with. For most applications, this is easy because they use some standard toolkit like GTK or Qt, which will transparently use Wayland without the application caring about it. But Firefox uses its own toolkit, so it's a bit more work.