Firefox getComputedStyle

Firefox getComputedStyle['borderWidth'] is null.

HTML

<div id="test"></div>

CSS

#test{
 width: 96px;
 height: 96px;
 border: solid 2px black;
 background: blue;    
}

JavaScript

/*
On Firefox 15.0 cStyle['borderWidth'] returns NULL on a borderd div.
Differently borderBottomWidth, $$leftWidth and $$rightWidth return correct value. I use only borderTopWidth as example.
*/

var elm = document.getElementById("test");
var cStyle = window.getComputedStyle(elm, null);

var bw = cStyle['borderWidth'];
var btw = cStyle['borderTopWidth'];

alert("borderWidth: "+bw+" - borderTopWidth "+btw);