scratchpad.js

var windowMediator = Cc["@mozilla.org/appshell/window-mediator;1"]
  .getService(Ci.nsIWindowMediator);
let top = windowMediator.getMostRecentWindow("navigator:browser");
let browser = top.gBrowser.selectedBrowser;
let chromeWin = browser.contentWindow.QueryInterface(Ci.nsIInterfaceRequestor)
                       .getInterface(Ci.nsIWebNavigation).QueryInterface(Ci.nsIDocShell)
                       .chromeEventHandler.ownerDocument.defaultView;
let windowUtils = chromeWin.QueryInterface(Ci.nsIInterfaceRequestor)
                     .getInterface(Ci.nsIDOMWindowUtils);
chromeWin;