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.
Check your enhanced Tracking protection settings and see if its at strict. Another user had the same problem here: https://lemmy.ml/post/3612554
It works perfectly on my end: