--- a/testing/marionette/element.js
+++ b/testing/marionette/element.js
@@ -20,17 +20,19 @@ const {
 } = Cu.import("chrome://marionette/content/error.js", {});
 Cu.import("chrome://marionette/content/wait.js");
 
 const logger = Log.repository.getLogger("Marionette");
 
 this.EXPORTED_SYMBOLS = ["element"];
 
 const DOCUMENT_POSITION_DISCONNECTED = 1;
+
 const XMLNS = "http://www.w3.org/1999/xhtml";
+const XULNS = "http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul";
 
 const uuidGen = Cc["@mozilla.org/uuid-generator;1"]
     .getService(Ci.nsIUUIDGenerator);
 
 /**
  * This module provides shared functionality for dealing with DOM-
  * and web elements in Marionette.
  *
@@ -941,18 +943,17 @@ element.isKeyboardInteractable = functio
  */
 element.scrollIntoView = function(el) {
   if (el.scrollIntoView) {
     el.scrollIntoView({block: "end", inline: "nearest", behavior: "instant"});
   }
 };
 
 element.isXULElement = function(el) {
-  let ns = atom.getElementAttribute(el, "namespaceURI");
-  return ns.indexOf("there.is.only.xul") >= 0;
+  return el.namespaceURI === XULNS;
 };
 
 const boolEls = {
   audio: ["autoplay", "controls", "loop", "muted"],
   button: ["autofocus", "disabled", "formnovalidate"],
   details: ["open"],
   dialog: ["open"],
   fieldset: ["disabled"],