Found from https://support.mozilla.org/en-US/questions/1364459

You can look at these prefs on the the about:config page to see what settings work for you to block autoplay.

media.autoplay.default = 5 [0:allow;1:blockAudible;2:Prompt;5:blockAll]
media.autoplay.blocking_policy = 2
media.autoplay.allow-extension-background-pages = false
media.autoplay.block-event.enabled = true