Hi, the settings have evolved.

The old media.autoplay.enabled preference has been replaced by several new ones. Let's give this a try:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful or accepting the risk.

(2) In the search box above the list, type or paste autop and pause while the list is filtered

To set your preferred behavior:

Allow autoplay by default (default for Release)

• Double-click the media.autoplay.default preference and set it to 0 (or right-click > Reset)

Block autoplay by default

• Double-click the media.autoplay.default preference and set it to 1

=> Does that work for you?

Ask on a site-by-site basis (default for "Nightly")

• Double-click the media.autoplay.default preference and set it to 2
• Double-click the media.autoplay.ask-permission preference to switch the value from false to true
• Double-click the media.autoplay.enabled.user-gestures-needed preference to switch the value from false to true

I'm sure we'll learn more about the interaction of these various preference and what sites can bypass them as more users begin to run Firefox 63.

See also:

• https://support.mozilla.org/en-US/kb/block-autoplay

When these two prefs are true (or at least user-gestures-needed) then there is also a setting available in "Options/Preferences -> Privacy & Security -> Permissions" and you can inspect and modify the exceptions.

• media.autoplay.ask-permission = true
• media.autoplay.enabled.user-gestures-needed = true