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Get uMatrix and block Media requests globally, then whitelist on a per-domain basis. That should "fix" auto-playing videos for you permanently.

This is built in to Firefox now, the change to the preferences the OP is talking about was implementing that feature.

That sounds like a lot of work for something that a browser should provide an easy, up-front way to configure.

But it's really not. Different users have different preferences and it seems like you would love uMatrix. I would suggest you give it a try.