From the people I talked to it seems that the main audience for firebase is mobile engineers that either don't have the skillset or the time to learn how to build a backend system. Most people I talked to also told me they wouldn't use firebase if they could build a backend.
I could see Firebase being a great go to for a certain size of project in the small to medium size that would rather pay for this feature set that using AWS, Azure, Rackspace, etc.
In some circumstances, interacting with users via the database as a messaging system can protect them from issues with your server code, and server outages: if your server goes down, users will still be able to log in and interact with the database via client side code - they'll only lose access to server related functions.
I've added an issue to document the process for uploading to firebase after building a production version.