Great add-on! Recently I started having trouble with chat ("Please disable popup blockers and try again." but the popup window showed up fine, but the login itself didn't work). I tried a lot of things (disabled built in popup blocker, allowed all cookies, allowed an exception for twitch cookies, disabled all other add-ons, ... ).
In the end I found a fix, which was turning "privacy.firstparty.isolate = false" in my about:config settings.
I had set this to "true" a long time ago (I found this in a privacy guide) and it never was a problem for the alternate twitch player / twitch chat. But apparently something changed recently, so it became a problem. But now that I set it to "false" again, twitch chat works well again.
edit: Thanks for the reply, yes, you're probably right, I didn't think of that.