So firefox is a class, and there are instances of that class running. Most instances are running on mobile devices, so it really makes sense to say that only a small minority of browsers are capable of running WebGL.
I'd say most of mobile browsers can run WebGL, it's just that the experiences are not designed for mobile platforms (this Polygon Shredder, for instance, it does work on mobile, it's just too slow to be good, but I can probably make it work)
So firefox is a class, and there are instances of that class running. Most instances are running on mobile devices, so it really makes sense to say that only a small minority of browsers are capable of running WebGL.