I have this problem with my rollover images in CSS with the Firefox 3 browser. I made one image in photoshop and I have it shifting positions in CSS when I rollover it. The problem is, the box is overlapping the rest of the links making them unclickable. This problem only exists in Firefox 3, it works fine in Firefox 2, IE, and Safari.
Take out the display: block; and set that to display: inline; as they are inline and not blocked. Once you do that, add float: left; to your a elements and that should fix the problem. If not, it will set you on a path to get it working!