just setup a private #firefox #sync server. pretty straight forward... :firefox:
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