I had the same problem as Op.
for over a year i couldnt log into my twitch account cause it kept pissing and moaning about my browser not being supported, despite being on a supported version and browser.
persisted through reformats, through LiveCD tryouts, different physical computers, browser resets/reinstalls, and with all addons uninstalled and/or disabled.
Only recently started working again for some reason.
I’d wager twitch is just shit at shit.
thank god I wasnt a streamer wasnt financially dependent on it.
the only two things that shock me about this is
That it took until now for it to happen
that people are shocked by it.