For most applications it's usually enough that the toolkit (GTK, Qt, whatever) supports it. However, Firefox probably does some stuff 'outside' what GTK provide

For most applications it's usually enough that the toolkit (GTK, Qt, whatever) supports it. However, Firefox probably does some stuff 'outside' what GTK provides, so it needs to implement those bits itself. I'm guessing stuff like <video/> support. (Wayland is a set of protocols and the toolkit may not support all the protocols or have sub-par support -- in which case the application can choose to do those bits itself.)