I am guessing this has something to do with your OS or gfx card more then your Firefox. It seems the error is most likely due to something stopping WebGL initialization on your Firefox.
On a brand new installation of FF dev on Windows it works.
Yea, pretty sure theres some webgl config settings that I need to tweak - works fine in Chrome, I just use that as fallback in case something doesn't work in FF.
On a brand new installation of FF dev on Windows it works.