Apparently the old setting
media.autoplay.enabled, which stopped autoplaying videos in Firefox, has been deprecated in version 63 and no longer works. This setting has been moved to
media.autoplay.default. Setting it to
1 disables autoplay,
0 enables it, and
2 asks for permission per site.
My configuration is now:
media.autoplay.default: 2 media.autoplay.ask-permission: true media.autoplay.enabled.user-gestures-needed: true
This stops autoplaying videos on all sites I have encountered so far. You can read more in this question on the Mozilla support site. As of
now this needs fiddling with
about:config, but the developers plan to integrate it in the settings UI in the future.