How is Horizon different from Firebase?
There are a few major differences:
Horizon is open-source. You can run it on your laptop, deploy it to the cloud, or deploy it to any infrastructure you want.
Horizon will allow you to build complex enterprise apps, not just basic applications with limited functionality. Since Horizon stores data in RethinkDB, once your app grows beyond the basic Horizon API, you can start adding backend code of arbitrary complexity that has complete access to a fully-featured database.
Since Horizon is built on RethinkDB, we'll be able to expose services that are much more sophisticated than simple document sync (e.g. realtime analytics, streams on joined tables, etc.)