User Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:80.0) Gecko/20100101 Firefox/80.0
Steps to reproduce:
Upgraded FF on Windows 10 and Linux Mint Ulyana (20) to 80. Tried to continuously use my own sync server hosted on own server that worked flawlessly with FF < 80.
Menu shows yellow warning sign and requests me to log into my FF account. After login, yellow warning sign shows again, requesting me again to log into my FF account, and so on.
Additionally (and maybe caused by the login problem), syncing bookmarks via my own sync server has stopped working. It still works on my office PC with FF < 80.
FF should acknowledge a successful login into my FF account and should sync my bookmarks via my own sync server, as it worked before upgrading FF to 80.