Refer: https://support.mozilla.org/en-US/questions/1222694
about:config?filter=autoplay
Helpful Reply
The pref that controls auto-play has been changed in the current Firefox release (63.0.1) to an Integer value.
media.autoplay.default = 0
When the first two prefs listed below (or at least user-gestures-needed) are true then there is also a setting available in "Options/Preferences -> Privacy & Security -> Permissions" and you can inspect and modify the exceptions.
media.autoplay.enabled.user-gestures-needed = true
media.autoplay.ask-permission = true
media.autoplay.default = 0 [0=Allowed, 1=Blocked, 2=Prompt]
You can open the about:config page via the location/address bar. You can accept the warning and click "I accept the risk!" to continue.
http://kb.mozillazine.org/about:config
See also:
https://support.mozilla.org/en-US/kb/block-autoplay