Example of building a basic "index" in Firebase.

gistfile1.js

var users = firebase.child('users');
var usersByEmailIndex = firebase.child('indexes/users/email');

var createUser = function(username, emails) {
  var newUserRef = users.push({
		username: username,
		emails: emails
	});
	var userId = newUserRef.name();
	emails.forEach(function(email) {
		usersByEmailIndex.child(firebaseCodec.encode(email)).set(userId);
	});
	return userId;
};

var getUserByEmail = function(email, callback) {
	usersByEmailIndex.child(firebaseCodec.encode(email)).once('value', function(snapshot) {
		var userId = snapshot.val();
		users.child(userId).once('value', function(snapshot) {
			callback(snapshot.val());
		});
	});
};

createUser('Erik Beeson', ['erik1@example.com', 'erik2@example.com']);
getUserByEmail('erik2@example.com', function(user) {
	console.log(user); // {username: 'Erik Beeson', emails: ['erik1@example.com', 'erik2@example.com']}
});