• about:config
  • Search for media.autoplay
  • Set media.autoplay.blocking_policy to 1
  • Set media.autoplay.default to 5

Bonus: How to set default volume level: media.volume.scale set to .25 (or other sub 1 number)

• Enter "about:support" as URL and select the "Open containing folder" button. You can reach it using Menu "Help" -> "Troubleshooting Information", too:

• MacOS default path: ${HOME}/Library/Application Support/Firefox/