http://stackoverflow.com/questions/430485/ie-css-bug-table-border-showing-div-with-visibility-hidden-position-absolute
This is a IE bug. Firefox doesn't recognize "border-collapse" using "border-spacing" instead which does not cause this problem. The solution of using "display:none" works, but there's another possibility. If the visibility property is set using Javascript then the border is hidden as well (as expected).