The ability to stream media from legit paid sources. (Netflix, Comcast, max, disneyplus, prime, I don't know where the list is currently, but anything that bitches about user agent.)

Agreed, that's critical. That said, I periodically subscribe to all of those, and all of the ones I've tried in the last year on Firefox on Debian, have worked perfectly. If there's any left that still don't, I haven't tried encountered them.