Can't login to gitlab.com on linux with firefox (422)

I have a Linux machine (manjaro) and firefox 83.0 - but I can’t use it to login into gitlab.com, every time I get error 422 (The change you requested was rejected.). Cache/cookie cleaning, firefox reinstalling takes no effect. In chromium everything works well, no problem with signing in. The problem was on earlier firefox versions, too. I don’t know where to dig to find a solution. Reinstalling distro is not an option. Looks like a hidden ban from gitlab.com server, but I have no clue about reason.
Sorry if I choose wrong category.

Try incognitio/private mode in firefox. Maybe you have a plugin installed that is causing problems. I have Linux Mint and Firefox 83 and it works fine for me. So you must have some issue inside your browser with extensions or something.

Alternatively disable all extensions/plugins to see where the problem is.

I have started Firefox in safe mode without extensions and opened private window and it doesn’t help.

One other thing you can try from your home directory:

mv .mozilla .mozilla.old

then run Firefox again so that it creates a new empty profile and see if that helps. Make sure first that .mozilla doesn’t exist as a directory, you can do:

ls -lha | grep mozilla

to verify this. It should only show .mozilla.old - then run Firefox and see if gitlab now works.

I tried it, but it doesn’t help. I’m still getting 422 error. Interesting moment that in chromium I have a little delay after pressing “sign in” button and before actual signing in (page takes time for loading), but in Firefox I get 422 page almost immediately.