In my limited testing it looks like I can finally watch 4K videos again! My system can handle 4K video, but apparently not 60 fps. Funny thing is in a chromium based browser there is no stutter, and I heard a rumor that if you change the user agent in firefox to chromium it would remove the stutter. You can specify sites to use a different user agent in firefox, you don't have to do it globally.

Is it just me or did google remove the youtube html5 player test as shown in the screenshot?