They are blocking the people using the adblocking features. But normally they have to show a message, I think YouTube is testing this "feature".
Firefox
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Videos do not play if they are part of a playlist,Youtube works normally when i use librewolf(with ublock) though
I have no clue what is causing the problem. But can you try the below steps?
I would suggest you to create a new profile by going to about:profiles and opening youtube.
Or you can maybe try to refresh the firefox installtion.
I have the same problem, I just use freetube for YouTube now plus the quality stays the same unlike YouTube's auto 640p
https://discussion.fedoraproject.org/t/cant-play-videos-in-firefox/79645/9
You need to install proprietary stuff. Basically just get the rpm fusion repositories and download the stuff. Or you can just download the flatpak