A Firefox Javascript bug?

I just came over a strange behaviour in Mozilla Firefox. It seems Firefox ignores return values for dynamically added event handlers. I stumbled upon this, when I tried to implement some quick mechanism to cancel a form submission.

The solution is well known and looks like this:

// The java script
function cancel()
{
  return false;
}
 
// The form
<form onSubmit="return cancel()">

This works right away in any browser I tried.

However, this changes, if I attach cancel() dynamically, using addEventHandler() . This looks like this:

//The javascript
addEvent(window, "load", init);
 
function init()
{
  form = document.getElementById("form1");
  addEvent(form, "submit", cancel);
}
 
function cancel()
{
  return false;
}
 
function addEvent(obj, evType, fn)
{
  if (obj.addEventListener)
  {
    obj.addEventListener(evType, fn, false);
    return true;
  }
  else if (obj.attachEvent)
  {
  var r = obj.attachEvent("on"+evType, fn);
    return r;
  }
  else
  {
    return false;
  }
}
 
//The form
<form id="form1">

While this behaves essentially like the first example in Internet Explorer, Firefox will submit the form whatever actually is returned by cancel(). I tried this on the most recent version 1.0.1. I prepared a test page, so you may have a look at the code and the behaviour yourself.

This is weird and seems to be a bug. Or did I get something wrong?

Update: I noticed there a quite a lot of people comming here from Google. Be welcome! The problem has been solved in the mean time, a solution can be found here.