#Open with mpv Addon to open videos with mpv (given youtube-dl installed as well as mpv).
Playback of youtube videos in Firefox on OSX without flash plugin installed is very resources consuming (looks like that HW acceleration doesn't work properly), so I decided to use mpv video player with working HW acceleration for this purpose.
jpm xpi
In about:config set xpinstall.signatures.required to false
Open @open-with-mpv-0.0.1.xpi in Firefox to install extension