What are some alternatives?
When comparing NATS and Firebase, you can also consider the following products
Socket.io - Realtime application framework (Node.JS server)
Android Studio - Android development environment based on IntelliJ IDEA
SignalR - SignalR is a server-side software system designed for writing scalable Internet applications, notably web servers.
Xcode - Xcode 4 is the new version of Apple’s powerful integrated development environment for creating great apps for Mac, iPhone, and iPad. Xcode 4 includes the Xcode IDE, instruments, iOS Simulator, and the latest Mac OS X and iOS SDKs.
Pusher - Pusher is a hosted API for quickly, easily and securely adding scalable realtime functionality via WebSockets to web and mobile apps.
PubNub - PubNub is a real-time messaging system for web and mobile apps that can handle API for all platforms and push messages to any device anywhere in the world in a fraction of a second without having to worry about proxies, firewalls or mobile drop-offs.