Well you clearly haven't used the standard available download (non-beta/nightly release) consistently through last year. Waterfox was using ESR 128 since October 2024, kept that base until finally upgrading to ESR 140 last August. So that's nearly a year of its base being out of date. So the user agent reported that number... sites really don't like that since they're looking at that for support.

https://www.waterfox.com/releases/6.5.0/ https://www.waterfox.com/releases/6.6.0/

Twitch only supports the last TWO versions of Firefox officially and will actively block logging in from older versions. So while you might be able to watch Twitch, if you aren't already logged in, you won't be able to login.

https://help.twitch.tv/s/article/supported-browsers?language=en_US

There are thousands of posts about it online for Waterfox other forks.