// this will be much more efficient than $watch()
app.factory('FilteredArray', function($firebaseArray) {
  function FilteredArray(ref, filterFn) {
    this.filterFn = filterFn;
    return $firebaseArray.call(this, ref);
  }
  FilteredArray.prototype.$$added = function(snap) {
    var rec = $firebaseArray.prototype.$$added.call(this, snap);
    if( !this.filterFn || this.filterFn(rec) ) {
      return rec;
    }
  };
  return $firebaseArray.$extend(FilteredArray);
});

app.controller('...', function($scope, FilteredArray) {
  var ref = new Firebase('https://kato-books.firebaseio.com/');
  $scope.data = FilteredArray(ref, function(rec) {
    return rec.author !== 'Stephen King';
  })
});