How to find out if you are in an iframe when responding to DOMContentLoaded event in Firefox Extention

So I am working on a Firefox Extension and did not want my overlay to load in iframe on a page. After searching around I found this little bit of code that will tell you based on  the DOMContentLoaded  event.

 

function isTopLevelWindow(event)

{

    var window_interface =
          window.QueryInterface(Components.interfaces.nsIInterfaceRequestor)      

          .getInterface(Components.interfaces.nsIWebNavigation)
          .QueryInterface(Components.interfaces.nsIDocShellTreeItem)
          .rootTreeItem
          .QueryInterface(Components.interfaces.nsIInterfaceRequestor)
          .getInterface(Components.interfaces.nsIDOMWindow);

    var doc = event.target;        

    for (var i = 0; i < window_interface.gBrowser.browsers.length; i++) {
        if (window_interface.gBrowser.browsers[i].contentDocument == doc) {
              return true; 
            }
    }     
    return false;

}