After upgrading to FF 80 (on Linux Mint 20.04 (Ulyana) and Windows 10), syncing bookmarks via my own sync server doesn't work any more. It seems that FF doesn't store my login into my ff account. After entering my credentials (and usually entering the six-digit code mailed to me), i get a short success information, but immediately afterwards, in the menu symbol, there's a yellow warning sign again, telling me to login again into my ff account.
I think it's not a matter with my sync server since on my corporate pc (with FF < 80), syncing still works. Additionally, i didn't change anything regarding the sync server, i've just upgraded FF to version 80.
See also this reddit thread (workaround proposed there works only for android version of FF, i think...)
Would be very happy for any help or workaround!