Yesterday (19/08/2023) everything worked fine. Today (20/08/2023) I can no longer login to Twitch using Firefox. I restarted browser and cleared cache. No change.
EDIT: I tried again after 30min and it works again. I have some privacy-oriented plugins but I don’t play with custom useragent.
If you dont mind whitelisting cookies for the twitch domain, you can allow finger printing, log in, choose to stay logged in for 30 days, then disable finger printing again. And then you’ll only have to worry about it once a month.