Firefox is supposed to block autoplay of videos that have audio unless the user has clicked to play a video on the same page. There is definitely an arms race for ads that find holes in the autoplay heuristics.
You might try setting the about:config settings “media.autoplay.default” to 5 and/or “media.autoplay.blocking_policy” to 2:
I want a reliable way to block autoplay of all videos, whether or not they have audio.