I run my own firefox sync server, and have since it was introduced. It's working fine for the current version of sync. There's a rather helpful guide for setting it up and there were a handful of alternative server implementations which have unfortunately fallen by the wayside.
Unfortunately this server is not compatible with the new sync protocol. It will continue to work while there is still old-sync support in the browser (a few versions after FF29 at least) but will eventually need to be upgraded to the new system.
https://docs.services.mozilla.com/howtos/run-sync.html