It works, but you have to click 3 times to play a video.
- Click once to open the video.
- Player loads and video can start briefly before the plugin can stop it, this is coming from the way YouTube optimizes video pages to start playing ASAP.
- Plugin stops the video and shows the 'unloaded' view.
- Click the unloaded video to opt-in to playing it, still doesn't play.
- Click again to play. I don't know why you have to click twice here, but that makes three total.

It would be nice if clicking a video from the subscriptions or algorithm list would start playing immediately, but using back/forward or revisiting an old tab would not auto-play.

It is not working when open video in new table.

Doesn't perform well enough for initial page load. There will be a brief second or two that the video would still play before it gets replaced by a non-playing video.

the best solution :
https://support.mozilla.org/fr/questions/1238091
in about:config you can set media.autoplay.default to 1 in order to block automatic playback on all pages or set it to 2 to decide on a per domain basis.

Somehow works - but always lets the video play for a couple seconds, then reloads the page and then disables the video on the reloaded page...