just setup a private #firefox#sync server. pretty straight forward...
1. build a #docker image from https://github.com/mozilla-services/syncserver 2. #deploy it somewhere 3. configure identity.sync.tokenserver.uri in #about:config to point to your new location 4. login at `about:preferences#sync` (you still need a #mozilla account, unless you also run a private account server) 5. sync and check about:sync-log for #error|s