lucy24 - 5:26 am on May 20, 2011 (gmt 0)
I think this may be a rendering glitch in Mozilla. I use Camino (Mozilla Lite), and have sometimes noticed similar behavior. So your users are probably accustomed to it and won't think there is anything wrong with your site ;)
Oh, and it's safer to specify border width in pixels, rather than "thick" or "thin", because otherwise the exact size is at the browser's discretion. (Same goes for color. If you need black borders, be sure to say so.) You can get some truly icky results if you leave it up to the browser.