gistfile1.js

var saveUser = function(user, cb) {
  var ref = new Firebase("https://<your-firebase>.firebaseio.com");
  var usersRef = ref.child('users');
  
  usersRef.child('count').once('value', function(snap) {
    var priority = snap.val() ? snap.val() : 0;
    usersRef.child(user.uid).setWithPriority(user, priority, function() {
      // on complete, update user count
      usersRef.child('count').transaction(function (current_value) { return (current_value || 0) + 1; });
    });
  });
}