After updating to FF56, youtube videos playing at 60fps no longer works properly. It drops most of the frames and the video is very jittery. It was not a problem prior to this update. I tested by starting FF in Safe Mode and the problem went away. I have 2 extensions, Greasemonkey and Ublock Origin. I restarted FF normally and disabled Greasemonkey and the problem happened again. I disabled Ublock and the problem continued.

What else is disabled in Safe Mode that could be causing this issue to occur? I have 3 plugins and they were all enabled even in Safe Mode. I tried enabling/disabling "use recommended performance settings" and nothing changed.