gistfile1.js

// Go to about:config, turn on devtools.chrome.enabled and dom.udpsocket.enabled
// Open the console with Ctrl+Shift+J
// Copy/paste that in the input field:
{
  var url = "http://foo.com/bar";
  Components.utils.import("resource://gre/modules/Services.jsm");
  Services.perms.add(Services.io.newURI(url, null, null), "udp-socket", 1, 0, 0);
}