Yes, I have allowed autoplay on the video sites, but it is now click-to-enable on all those others sites that are just using it for ads.

Go to about:config in Firefox, and change "media.autoplay.allow-muted" to false.

Chrome has shut down auto-play on any video with sound (excepting video specific sites) and they have a flag "Autoplay policy" which was partially successful, but not as effective as the blocking in Firefox.

And that's exactly the problem.. it used to be very hard to block just a few weeks ago.

Sorry, what do you mean? How can I block autoplaying yt vids in firefox?

You could get to it through about:config (so.. a little difficult for the common user to reach)

media.autoplay.enabled

media.autoplay.allow-muted

They then changed to

media.autoplay.default

media.autoplay.block-webaudio

media.autoplay.enabled.user-gestures-needed

And removed the previous parameters, which caused the videos to start playing automatically again, until you would configure the new properties (again, only though about:config).

Finally, they're now providing a menu option for achieving the same, which deserves to be applauded.

I use "Enhacer for Youtube" extension.

Yes, these kind of extensions just used to take advantage of the existing properties.