Thumbnails shift to the right in Firefox

The thumbnails are positioned perfectly between the left and right arrows, in IE7 and Chrome, but in Firefox 4, the thumbnails are shifted to the right so that the right arrow is covered up by the last thumbnail image and there is a big gap between the left arrow and the first thumbnail image. See: image attached

I've tried everything in ad_gallery.css to correct this with no luck.

Any help would be appreciated.

Thanks.