https://wiki.mozilla.org/Media/block-autoplay has some more additional information in particular about the media.autoplay.blocking_policy pref which allows restoring the older behavior of the play intent not being sticky.
Which is great for most stuff, but doesn't seem to stop facebook 'reels' from animating. They must do some trickery like loading a short animated image as a 'teaser' before rendering the actual video on click.
Using this since I can remember. But noticed another issue and still searching how to disable the video cache at all until I start to play the video and to limit it to 2-5 seconds (or 1 MB) during playing, though no chance so far.
And just another issue since few months: youtube page auto-reloads/replaces itself since few seconds after opening (even if the tab is not active). The title gets replaced from the original language into English auto-translation (EN is default for browser), but I'd like to see video titles in the original language.
I can't try it at the moment, but I'm not sure the page is actually reloaded. I suspect YouTube is an SPA and, when you click, the url change event is captured by a router and the content of the page is merely replaced.