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.
Must be something local to you or twitch CDN. I just tried on the same version of FF from West Europe and it worked.
Have you by chance changed your useragent?