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 wor

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-autoplaying-in-firefox/ 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.