EDIT: here is a nice article how to replace Firebase in your app with Node.js+MongoDb. It shows how much work you would have to do on your own, and explains, IMHO, why a startup (small app) should begin with Firebase (if real-time updates to clients are required) and proceed with MongoDb (in any case self-written solution) if the project keeps evolving
EDIT 2: after being acquired by Google Firebase now offers various perks on top of its basic features which you would struggle to build on your own:
For development
For growth
After using Firebase a considerable amount I've come to find something.
If you intend to use it for large, real time apps, it isn't the best choice. It has its own wide array of problems including a bad error handling system and limitations. You will spend significant time trying to understand Firebase and it's kinks. It's also quite easy for a project to become a monolithic thing that goes out of control. MongoDB is a much better choice as far as a backend for a large app goes.
However, if you need to make a small app or quickly prototype something, Firebase is a great choice. It'll be incredibly easy way to hit the ground running.
Firebase provides some good features like real time change reflection , easy integration of authentication mechanism , and lots of other built-in features for rapid web development. Firebase, really makes Web development so simple that never exists. Firebase database is a fork of MongoDB.
What's the advantage of using Firebase over MongoDB?
You can take advantage of all built-in features of Firebase over MongoDB.
In my experience, working with Firebase is a huge advantage if you are trying to do user management, database, messaging sort of app since all of these features are already well integrated.
Like others have said, if you're just focused on the database/querying aspect, stick to mongo.
Firebase is a suite of features .
I believe you are trying to compare Firebase Realtime Database with Mongo DB . Firebase Realtime Database stores data as JSON format and syncs to all updates of the data to all clients listening to the data . It abstracts you from all complexity that is needed to setup and scale any database . I will not recommend firebase where you have lot of complex scenarios where aggregation of data is needed .(Queries that need SUM/AVERAGE kind of stuff ) . Though this is recently achievable using Firebase functions . Modeling data in Firebase is tricky . But it is the best way to get you started instantaneously . MongoDB is a database. This give you lot of powerful features. But MongoDB when installed in any platform you need to manage it by yourself .
When i try to choose between Firebase or MongoDB(or any DB ) . I try to answer the following .
Apples and oranges. Firebase is a Backend-as-a-Service containing identity management, realtime data views and a document database. It runs in the cloud.
MongoDB on the other hand is a full fledged database with a rich query language. In principle it runs on your own machine, but there are cloud providers.
If you are looking for the database component only MongoDB is much more mature and feature-rich.
I will answer this question in terms of AngularFire, Firebase's library for Angular.
Tl;dr: superpowers. :-)
AngularFire's three-way data binding. Angular binds the view and the $scope, i.e., what your users do in the view automagically updates in the local variables, and when your JavaScript updates a local variable the view automagically updates. With Firebase the cloud database also updates automagically. You don't need to write $http.get or $http.put requests, the data just updates.
Five-way data binding, and seven-way, nine-way, etc. I made a tic-tac-toe game using AngularFire. Two players can play together, with the two views updating the two $scopes and the cloud database. You could make a game with three or more players, all sharing one Firebase database.
AngularFire's OAuth2 library makes authorization easy with Facebook, GitHub, Google, Twitter, tokens, and passwords.
Double security. You can set up your Angular routes to require authorization, and set up rules in Firebase about who can read and write data.
There's no back end. You don't need to make a server with Node and Express. Running your own server can be a lot of work, require knowing about security, require that someone do something if the server goes down, etc.
Fast. If your server is in San Francisco and the client is in San Jose, fine. But for a client in Bangalore connecting to your server will be slower. Firebase is deployed around the world for fast connections everywhere.
Firebase is designed for real-time updates. It easily integrates with angular. Both are NoSQL databases. MongoDB can also do this with Angular through Socket.io integration. Meteor.js also makes use of MongoDB with an open socket connection for real-time updates.
MongoDB can be run locally, or hosted on many different cloud based providers. Firebase, in my opinion is great for smaller apps, very quick to get up and running. MongoDB is ideal for more robust larger apps, real-time integration is possible but it takes a bit more work.
Source: Stackoverflow.com