Firebase has a much nicer API. It's organized around a realtime, ordered, tree datastore. This makes it much more akin to very granular database than a message bus. Spire is just an authentication service + channels. There is no state beyond the historical list of messages in a channel. That makes it a really thin layer over Socket.IO. Firebase solves a much harder problem of state synchronization.

Firebase allows you can listen to any part of the subtree and query particular children of a node. Data is synchronized in realtime, optimistically, and the events that fire allow you to easily tie your UI or Backbone model to the datastore. It also has critical features like transactions. This is much more flexible than what Spire.IO currently provides.