function feedburner(){
  var forms = document.getElementsByTagName("form");
  for(var i = 0; i < forms.length; i++)
   {
     var form = forms[i];
     if(form.getAttribute("action").substring(0, 25) == "http://www.feedburner.com")
     {
        form.target = "_blank";
     }
   }
 }

window.onload = feedburner;