Retrieving JSON from firebase storage

I am using D3js to parse some information and would like the JSON to be stored online rather than locally. I am pulling the JSON from firebase doing the following:

 var storageRef = firebase.storage().ref().child("main_bubble.json");
storageRef.getDownloadURL().then(function(url) {
  console.log(url);
});

it returns the URL properly, but how do I convert it into JSON?