I tried searching the forums for this but I couldn't find anything on it so:
Starting a couple of months ago I've been unable to login on Firefox, both on Windows and on Android.
Simply put the login button does nothing but refresh the page for me.
I've tried disabling all add-ons and also tried starting Firefox in safe mode on Windows but no difference.

Anything I can do to fix this?

That's odd. Anything different happen with firefox since it happened?

No, only had this issue.

I stupidly hadn't tried this before but in private browsing mode (with everything enabled/normal otherwise) it works fine. So I tried clearing cookies and such but it still doesn't work when browsing normally. I'm at a loss.

I can't reproduce this on Firefox 85 on Windows or Nightly on Android. It sounds to me like you may have some sort of configuration issue. Are cookies fully enabled? Do you have any extensions installed? Do you remember modifying anything in about:config?

Yeah I think it's something on my Mozilla account that has synced across my devices but I can't really think of what that could be.
Everything is close to default settings as far as I know, but since I'm able to log in while private browsing with all my extensions still enabled I don't know what setting or configuration would do this. I tried disabling the "enhanced tracking protection", clearing all cookies, cache, and site data, but I'm still unable to log in.

Edit: Able to log in now. I tried just about everything so I'm not completely sure what fixed it, but I disabled all syncing in my account before clearing all cookies and data again, then re-enabling all syncing. Could it have been a broken cookie or something that had synced?

That's possible. I generally recommend against syncing cookies across devices.