Example: Twitch still doesn't let me login from Firefox if I'm on Alpine Linux.
It only happens when I try from Alpine Linux, so they might be testing for some musl-specific quirk and denying access just for not being the Firefox from a different distro.
Brave is unsupported by a lot of websites, but almost always works (at least if you turn off shields).
"Unsupported" = "We don't test or develop with this browser in mind", not "This browser doesn't work." If that browser effectively mimics one that is supported, it usually works fine.
This news doesn’t mean that it just will stop working in Firefox, it’s just that they don’t test and they don’t care.