Created attachment 301647 [details]
emerge --info www-client/firefox
Latest version of firefox is not able to play smoothly most HTML5 1080p videos, event in a small sized player. For the selected video, even 720p is not that smooth. In fullscreen, it is merely freezing all the time, it's unwatchable.
When downloaded with the proper extension, the selected video plays smoothly in mplayer, fullscreen. (I have a very recent gfx *-9999 stack for AMD GPUs)
I am unable to reproduce your results here. Time for you to provide much more info on which drivers you are actually using.
Created attachment 301715 [details]
This describes my gfx stack. Directly from git (time is now).
Screen is 2500x1600. mplayer2 has no issue rendering the downloaded video fullscreen and smoothly.
If you need something from the mozilla team please cc us. I am unable to duplicate using official portage tree.