Asynchronous access to an array in Firebase

Here is my code:

var userRef = new Firebase("https://awesome.firebaseio.com/users/");

var tokenRef = userRef.child(key+'/tokens');
tokenRef.once('value', function(snapshot){

  var userTokenSync = $firebase(tokenRef);
  var userTokens = userTokenSync.$asArray();


  console.log(userTokens);
  console.log(userTokens[0]);

  for(var i=0, len = userTokens.length; i < len; i++) {
     console.log(userTokens[i]);
  }
  console.log('done');
})

This code gets the tokens of a user from firebase, and I just want to browse the tokens array.
Here is what the console gives me:

As you can see, I cannot access the array. Do you have any idea of how I could do this?
Thanks in advance.