Very unusual image resize problem, driving me nuts

Hi all, I have been working on a site and I have a problem. Site: http://www.urbanestyles.nl/testsite. I am using the E-commerce plugin which, with a few adjustments, works like a charm for me. I only have one very unusual problem with the product pictures in Firefox. In IE everything works fine. Does anyone have any idea what causes FF to resize the pics (different sizes everytime you reload). Maybe it is something I have overlooked but I just can’t find it.

Example FF
http://img166.imageshack.us/img166/364/ffjx9.jpg

Example IE
http://img166.imageshack.us/img166/6897/ieqp7.jpg