1.Activating HTML5 player
As of this writing it is not activated by default in Firefox. It has something to do about Media Source Extensions, not activated by default in Firefox.
Go to https://www.youtube.com/html5 and “Select the HTML5 player” to activate the HTML5 player.
2.Get the 1080p stuff
To get Full HD resolution you need to enable both Media Source Extensions in general and MSE + WebM in Firefox.
about:config in the address and Click “I’ll be careful, I promise”
– Type mediasource in searchbox. Double-click both media.mediasource.enabled and media.mediasource.webm.enabled in order to change their value to “true”.
Update: the following steps is no longer needed as they conflict with FF 43 default setting.
and that’s it. You should get the option for 1080p in Firefox now.
How to get 1080p in Youtube’s HTML5 player in Firefox (on Linux) | LinuxVeda
HTML5 and 1080 in Firefox | PCLinuxOS forum