What site will support what browser isn't up to Firefox browser/Mozilla to decide that is what the site decided. Did you contact Twitch support to get more info as to why?

Firefox is most definitely supported by twitch. They themselves say they support the latest 2 versions of Firefox. It worked in the past and it was the recent Firefox update that started these issues. This is a Firefox issue, not a twitch issue. I did a clean reinstall of Firefox and installed the previous version, and there I could log in.

My experience: I tried to sign up and got that message, but then I clicked the Register button again and it sent me a code. Now I'm in.

I opened a private windows and logged in. No error.

Maybe there is something a little bit wrong with the scripts on the site at the moment, but since it isn't repeatable for me, it's very difficult to debug.

I did try private mode too, disabled all addons, even enhanced tracking protection, and put the security to the lowest setting. But it didn't work for me. I did not try to click register, but since I did all this voodoo stuff to log back in (reinstalls of an older version, new profile etc) I am not inclined to log out again to try :D But it may help someone.

Maybe I should add, that even with installing an older version, where I could log in, as soon as I logged into my Firefox account (to synch everything back again) I got the same issues again.

Sync doesn't transfer cookies or cached files, but maybe it brought over a setting or add-on that affects the site somehow?

I wouldn't know which. I have tried disabling all addons, like I said. And the settings I do think affect websites, like the security setting, or enhanced tracking protection I did try to disable too. Do you have any other ideas for settings that may affect a website? I'd like to try those too.