Strange dynamic menu border problem in Firefox

[sthrudel]

I'm using ASP Menu control. Orientation is set to Horizontal so the
dynamic part of the menu is shown below static menu.

I set the CssClass of the DynamicMenuStyle
(menu.DynamicMenuStyle.CssClass = "DynamicMenuStyle"


and this is in the css file:

..DynamicMenuStyle
{
border: solid 2px white;
}


When I'm viewing the page using IE the border of the dynamic menu is OK
(nice solid border around the dynamic menu), but when I'm viewing the
page using Firefox the bottom border's width is 4px. If I only set the
top border (border-top: solid 2px white), Firefox automatically adds
the bottom border also! Why is that?