Synching old Firefoxes with the new Firefox
The new Firefox on my freshly installed laptop only establishes connections with the v1.5 sync server, but my various other Firefoxes (most of them up to date too) still happily sync via the old 1.0 sync server.
Solution: Install an old Firefox, like say 21.0. Connect it to the old sync server. Then throw it away and use the new Firefox.