<html> <head> <script> onload = function() { var x = null; try { x = document.createElementNS("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul", "spacer"); } catch(ex) {} window.parent.postMessage(x == null, "http://mochi.test:8888"); } </script> </head> <body> Here be dragons! </body> </html>