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 use LibreWolf, which is FF based, and they refuse to let me log in. All it takes is a User Agent spoofer set to Chrome, and it works.
This is the way. User agent spoofer comes handy whenever you browse with FF