Firefox Sync was in the browser before, you just didn't need to sign in to your browser online to use the previous version. (It also let you sync through your own server rather than Mozilla's which I don't think the new version does yet, but even if you used their server you still didn't need a Mozilla account of any kind.)
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.
(edit: actually IIRC there's an open bug around allowing android devices to use a custom sync server; desktop devices definitely work fine with it via some about:config settings)
