Following up. I was doing it wrong. They changed the config schema for this stuff as of FF63. This post explains how to use the new controls properly, which worked for me: https://www.howtogeek.com/341690/how-to-prevent-videos-from-...

I have now a permission pop-up to allow autoplay, which remembers pref on a per site basis. Btw this is FF64, from ubuntu repo:

$ apt policy firefox

firefox:

  Installed: 64.0+build3-0ubuntu0.18.04.1

GLHF.