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.
I have had this exact issue for so long now. What always works for me is a simple reload.
Also unrelated, but still want to say it: Fuck every single browser based on chromium!