My feeling is that both wants to solve the exact same problem but they decided of a different route to get there. I feel the Firebase way started more as marketting and hype, while Spire.IO is a more traditional minimalistic version of what the real thing could be. (For instance, I could write a real app on spire while it'd be impossible on firebase as there's no authentication system.)
I think Firebase will eventually figure out how to pull off their service. After all, they aren't planning on leaving beta until the end of the year (according to their support rep). It is frustrating though. I really dislike when companies promise solutions to problems they haven't even figured out yet.