First, here are some relevant resources and suggestions:

Resources

• Check out Firebase's blog post, Best Practices: Arrays in Firebase - "Arrays are evil".
• Also, the AngularFire Development Guide.
• And the documentation for AngularJS providers.

Suggestions

As the AngularFire API Documentation says:

"There are several powerful techniques for transforming the data downloaded and saved by $firebaseArray and $firebaseObject. These techniques should only be attempted by advanced Angular users who know their way around the code."

Putting all that together, you accomplish what you want to do by:

• Extending AngularFire services, $firebaseArray and $firebaseObject.
• Following the documentation for extending services.

Example

Extended Error $firebaseObject

.factory('Error', function(fbUrl, ErrorFactory) {
  return function(errorKey){
    var errorRef;
    if(errorKey){
      // Get/set Error by key
      errorRef = new Firebase(fbUrl + '/errors/'+errorKey);
    } else {
      // Create a new Error
      var errorsListRef = new Firebase(fbUrl + '/errors');
      errorRef = errorsListRef.push();
    }
    return new ErrorFactory(errorRef);
  }
})

.factory('ErrorFactory', function($firebaseObject){
  return $firebaseObject.$extend({
    sendReply: function(replyObject) {
      if(replyObject.message.isNotEmpty()) {
        this.$ref().child('replies').push(replyObject);
      } else {
        alert("You need to enter a message.");
      }
    }
  });
})

Error Controller

.controller('ErrorController',function($scope, Error) {
  // Set empty reply message
  $scope.replyMessage = '';

  // Create a new Error $firebaseObject
  var error = new Error();
  $scope.error = error;

  // Send reply to error
  $scope.reply = function(){
      error.sendReply({message:$scope.replyMessage});
  }
})

And String.prototype.isNotEmpty()

String.prototype.isNotEmpty = function() {
    return (this.length !== 0 && this.trim());
};

(adapted from this answer)

Hope that helps!