controller.js

app.controller('ctrl', function(posts, $scope) {
  var postId = 'post123';
  posts.$loaded(function() {
    $scope.post = posts.$getRecord(postId);
  });
});

posts.js

app.factory('Posts', function(FIREBASE_URL, Firebase, $firebase) {
  var ref = new Firebase(FIREBASE_URL + 'posts');
  return $firebase(ref).$asArray();
});