code: function(model, options) {
            model.id = Backbone.Firebase._getKey(this.firebase.push());
            options = _.extend({ autoSync: false }, options);
            return Backbone.Collection.prototype.create.apply(this, [model, options]);
          }
docs: Create an id from a Firebase push-id and call Backbone.create, which
  will do prepare the models and trigger the proper events and then call
  Backbone.Firebase.sync with the correct method.