Firefox's built-in "Block Audio and Video" in Settings -> Autoplay seems to do a good enough job for most sites (with exceptions whenever it breaks something).
Does that help with "auto"play & PIP thats triggered by scrolling? I have not managed to completely deactivate PIP videos in Firefox settings/about:config.
I know of a very popular newswebsite in my region which somehow works around this: When overscrolling the video window on their site, ALWAYS the video starts to play - I have the option disabled in FF settings, nontheless its not working in this case :-X