I'm working on an open source firebase-like database that puts realtime/sync/replication/master-master/p2p at its core, rather than an after thought which a lot of other databases (MySQL, MongoDB, etc.) do. The RethinkDB guys are smart and I'm glad they are pushing in this direction, it is important. My project is at http://github.com/amark/gun if anybody is interested.
(I mean, nothing wrong with a project that duplicates others, but it seems odd to focus on master-master replication without distinguishing yourself from the most popular DB that's already built around that.)