I had to clear my browser cache for it to work

Clear your cache, turn it off and on again and eat all the cookies

https://github.com/LemmyNet/lemmy-ui/issues/1146
Known issue with lemmy for what looks like a month. Safari works but Firefox doesn’t despite using WebKit/Safari engine. Some in comments mentioned that it might be related to auto-fill. One got it working by deleting and adding one character. Another typed everything in manually. One had to switch from using username to using email address to login. Check out the thread and try some of these. Might have to use Safari until upstream fixes if one of the other comment suggestions don’t work.

I still can't get Fennec to work. The little bear keeps spinning after I click the login button

Edit: nevermind it works if I use my email for the username