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

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?