Change window.onbefureunload implementation due to event specification?

Firefox documentation (https://developer.mozilla.org/de/docs/Web/API/WindowEventHandlers/onbefo...) says:

window.onbeforeunload = function(e) {
  var dialogText = 'Dialog text here';
  e.returnValue = dialogText;
  return dialogText;
};

is correct. e.returnValue should have the text assigned. Currently the modules implementation is:

window.onbeforeunload = function() {
            return msg;
          };

I ended up this way:

window.onbeforeunload = function(e) {
            e.returnValue = msg;
            return msg;
          };