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.
General rule of thumb: If you suddenly encounter issues with your webbrowser, always check with another, clean profile. Preferably without any extensions.
Especially site-specific ones.
Glad you got it working again.