var Cc = Components.classes;
var Ci = Components.interfaces;
var observerService = Cc['@mozilla.org/observer-service;1'].getService(Ci.nsIObserverService);
var httpResponseObserver =
{
  observe: function(subject, topic, data)
  {
    if (topic == "http-on-examine-response") {
      var httpChannel = subject.QueryInterface(Components.interfaces.nsIHttpChannel);
      httpChannel.setResponseHeader("Access-Control-Allow-Origin", "", false);
    }
  }
};

observerService.addObserver(httpResponseObserver, 'http-on-examine-response', false);