WebGL works fine for me on latest Firefox stable (v43) and Nightly on an nVidia GeForce GTX 660.
Browser makers regularly update their GPU blacklists as they find out about more issues - it might be that your driver/hardware was added to the blacklist for this version. Also sometimes there are mistakes and more devices are blacklisted than they intended, so it could be that. IIRC, they can update the blacklist separately to doing a full browser release.