This I do not know, it is not offered on Macs or other browsers I have tested either. I would suggest to ask Youtube. https://addons.mozilla.org/en-US/fire.../youtube-high-definition/-> this add on will offer automatically detecting HD however.

I get 720p60 and 1080p60 offered in the current Nightly build that has MSE enabled by default.

• media.mediasource.enabled = true
• http://www.youtube.com/html5

Otherwise Google offers the 60fps choice via 720p60 and 1080p60 only to Google Chrome.

• https://www.youtube.com/playlist?list=PLbsGxdAPhjv9UrLo19pS8teoRKj7funAy

It doesn't work smoothly though and there are frames not painted for me on Linux.