Firefox and keydown event inside DIV elements

[AsGoodAsItGets]

Hello,
can anybody please explain to me why the following code does not work in Firefox as it does in IE?
In general, how can I capture keydown events inside DIVs with firefox?

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
  <title>keydown test</title>
</head>
<script language="JavaScript" type="text/javascript">
function keydown(evt)
{
     obj = document.getElementById("test");
     if(obj)
     {
          if(window.event)
               var key = evt.keyCode;
          else if(evt.which)
               var key = evt.which;
          else
               return true;
          obj.innerHTML += "user pressed '"+String.fromCharCode(key)+"' ("+key+")<br>";
     }
}
</script>
<body>
<div id="test" style="border: 1px solid black; padding: 8px; position: absolute; overflow: visible; left: 0px; top: 0px; -moz-box-sizing: border-box; width:400px; height:100px; font-family:tahoma; font-size:11px; background-color:#CCCC66;" onkeydown="return keydown(event);">
</div>
</body>
</html>

Thanks in advance.