I made an image thumb gallery in my forum with a unordered list and background images for the list items i.e. thumbs.

The result:

http://forum.dutchbodybuilding.com/ (just under the forum welcome message).

The problem is in Firefox, when you hover over the thumbs, they move to the left or right, making the borders look ugly. This problem doesn't show in IE.

Any clue about why this happens?

Thanks - dutch