As with any commercial provider, you don't have any guarantee here. This may be even more so now that Google has acquired Firebase.
Maybe you build an awesome app with Firebase, and in 12 months, Google decides that it just wanted the technology and the team, and shuts down Firebase as an independent service. What will you do?
As far as I can see, the Couchbase project offers similar features and all pieces of the stack are open-source.
How does this help? We can just re-use Couchbase Server and Couchbase SyncGateway. And on top of Couchbase Mobile and their other client libraries (iOS, Android, .NET), one can build a "shim" that maps Firebase's API to the Couchbase API.
The result will be a drop-in replacement for Firebase that is 100% open-source.
Maybe it will be most efficient to continue using Firebase even then. But it's always good to have a safe replacement up your sleeve.
