I'm running Firefox 32 Beta on Windows 8.1 with DPI scaling, in case that helps narrow it down. Tried to test it, but I don't think I can get FF to stop scaling without logging out and back in; quitting and rerunning it with the disable scaling box checked doesn't work. That's normally the first thing I look at for graphical glitches on here, though I have no idea how it affects WebGL.
Great job with this! Coming from Google Maps and similar, the smooth motion is mesmerizing, especially labels staying level as you rotate. And tile loading/pop-in is much quicker than what I get from Google.
https://github.com/mapbox/mapbox-gl-js/issues/652