There's a few children of this comment, noting this, but to tie things together from someone from Mozilla:

- You can self-host your own sync server. It's not something we spend a lot of time making easy, but it is possible and supported in some capacity: https://mozilla-services.readthedocs.io/en/latest/howtos/run...

- You can definitely build extensions for other browsers (or a standalone app) that implement the Sync protocol.