Kind of bummed it wasn't working for me. On Win 10 here with Firefox 76. I got the native client working and when on a youtube video, the toolbar icon for Send to MPV player would first start playing audio from the video, then a few seconds later the video (MPV) would finally open, then it just stops playing sound or video. :( ~ MPV player works just fine playing a youtube video by dragging the url into the player. Oh well :\
https://addons.mozilla.org/en-US/firefox/addon/play-with/ works flawlessly and doesn't require the native client.
https://addons.mozilla.org/en-US/firefox/addon/ff2mpv/ is another option, though I haven't tried it.

the toolbar button works fine but the right click context-menu has a weird behaviour and somtimes crashes.