Really stop autoplay in Firefox / Librefox

I am not sure who thought, that autoplay on music and videos is a good idea in a browser. Using my tablet on travel, it is just a joy to click on a link and some video starts entertaining everyone else but me. Firefox offers some toggles in the regular settings to deactivate autoplay, however I found, that it does not stop videos from playing in certain situations.

So what is the solution

Open a page with about:config and set the following settings:

This works for me in Firefox / Librefox and Firefox Mobile (nightly)