[uallo]

Settings => Search for "Autoplay" => Click "Settings..." => For "Default for all websites" select "Block Audio and Video".

[Semaphor]

Thank you, I could have sworn I had that active, but for some reason it was "audio only".

[styanax]

The about:config settings which you can look up:

    media.autoplay.blocking_policy
    media.autoplay.default

I have mine set to 2 and 5 respectively.

[slumberlust]

Thanks, I also noticed they allowlisted the FF about page...which is a bit cheeky.