--- a/toolkit/devtools/LayoutHelpers.jsm
+++ b/toolkit/devtools/LayoutHelpers.jsm
@@ -10,22 +10,24 @@ const Cr = Components.results;
 
 Cu.import("resource://gre/modules/XPCOMUtils.jsm");
 
 XPCOMUtils.defineLazyModuleGetter(this, "Services",
   "resource://gre/modules/Services.jsm");
 
 this.EXPORTED_SYMBOLS = ["LayoutHelpers"];
 
-this.LayoutHelpers = LayoutHelpers = function(aTopLevelWindow) {
+let LayoutHelpers = function(aTopLevelWindow) {
   this._topDocShell = aTopLevelWindow.QueryInterface(Ci.nsIInterfaceRequestor)
                                      .getInterface(Ci.nsIWebNavigation)
                                      .QueryInterface(Ci.nsIDocShell);
 };
 
+this.LayoutHelpers = LayoutHelpers;
+
 LayoutHelpers.prototype = {
 
   /**
    * Get box quads adjusted for iframes and zoom level.
 
    * @param {DOMNode} node
    *        The node for which we are to get the box model region quads
    * @param  {String} region