Creating an iframe in Firefox extension code

## firefox_ext_iframe.js

if (Components && Components.classes) {
  // This means we are in a Firefox extension.
  var appShell =  Components.classes["@mozilla.org/appshell/appShellService;1"]
                  .getService(Components.interfaces.nsIAppShellService);
  // hiddenDoc will now point to a regular DOM document.
  var hiddenDoc = appShell.hiddenDOMWindow.document;

  // Create an iframe.
  let iframe = hiddenDoc.createElementNS("http://www.w3.org/1999/xhtml", "iframe");
  iframe.setAttribute("mozframetype", "content");
  hiddenDoc.documentElement.appendChild(iframe);

  // Load a page in it.
  iframe.setAttribute("src", "http://google.com");
}