If you go with Rails you can strip it down and check http://api.rubyonrails.org/ for APIs. There's similar project for APIs in Django.
I had a look at Firebase and my main problem with it is having to rely on 3rd party service and storing data on their servers + portability if I was to decide to leave them in the future.
I've had some good mileage out of SailsJS ( http://sailsjs.com/ ) recently, essentially it is a NodeJS + Express stack with Socket.io and prebuilt ORM/blueprints. Up and running in a matter of minutes and behind the scenes it is still just NodeJS/Express
Tastypie lets you build APIs quickly, and the Django admin lets you browse your data without having to write any code at all. For the kinds of apps I build, things can't get easier than this. YMMV, of course.
I have had some minor issues with Tastypie, but my current project is too far along at the moment to switch to another framework.