Should work on Chrome, Firefox, Safari, IE, Opera, Edge... any that supports WebGL with OES_standard_derivatives (so basically 99% of browsers according to http://webglstats.com/)
What version of Firefox, OS, GPU, etc. are you using?
Technically, they're wrong because the stats are collected by trackers in sites about WebGL, so there's a bias towards having complete WebGL implementations.
Of course there are more than 1% browsers that support, but we're talking about implementations, not devices. You can can have an old iPhone running iOS pre8 and it won't run WebGL, but one can say that iOS does support WebGL now.
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)
Firefox beta/Aurora/DeveloperEdition... all but stable have a regression on WebGL and reporting missing extensions support, like OES_texture_float. Most of the three.js recent content will probably not work on them.
What version of Firefox, OS, GPU, etc. are you using?