根据您的评论更新答案。

您想听/activity/$theid/thread/$threadId：

与其嵌套，不如嵌套/thread/$threadId在根中。

{
  "threadActivity": {
     "$activityId": {
        "$threadId" : {

        } 
     }
   } 
}

现在，当添加新线程时，您可以只听/threadActivity/$activityId：

var ref = new Firebase('https://<my>.firebaseio.com/threadActivity');
var activityRef = ref.child('some-id');
activityRef.on('child_added', function(snap) {
  // will update you for every child at the location and each child added
  console.log(snap.val()); 
});