It was indeed a Twitch update that broke FFZ and the extension already got updated so just use the userscript version.

FFZ has honestly been a such a fantastic tool to use for all these years but once again I'm here to check what's going on with the addon as even the userscript version stopped working so you can't even blame it on Mozilla anymore. Frankly maybe some Twitch update broke FFZ. A lot of people reporting in Github too.