[jQuery] .css("border-color") problem in FireFox

[wellmoon]

Hi,

I want to get the border-color of a div.  The following code works in
IE and alerts the color of the border:

$("#mydiv").each(function() {
  var borderColor = $(this).css("border-color");
  alert(borderColor);
});

In FireFox it just alerts 'undefined'

The border is set with CSS, not by javascript or jquery.  Am I doing
something worng or is this a bug?