Why is DomParser having issues in Firefox?

Question

I am writing a Greasemonkey script; it works great in Chrome, but has issues only in Firefox. I am using the following code to parse a response using xmlHTTPRequest:

var parser      = new DOMParser ();
var responseDoc = parser.parseFromString (response.responseText, "text/html");

So, I can do stuff like responseDoc.getElementById, etc. This works in Chrome, but I always get undefined in Firefox. This thread sounded kind of similar, but I am not too sure the solution works (if there is a solution - not super clear):

DOMParser().parseFromString() not giving response with Firefox

Any help would be great!

Edit, sample added:

GM.xmlHttpRequest({
  method: "GET",
  url: "https://www.google.com/",
  onload: function(response) {
    alert(response.responseText);

    var parser      = new DOMParser ();
    var responseDoc = parser.parseFromString (response.responseText, "text/html");

    alert(responseDoc.innerHTML);
  }
});