SlideShow: Images cropped in Firefox

This took me now my weekend. I am using slideshow on WP 3.5.1
Installation, uploads and anything else works fine. The only problem is, that on Firefox the images are cropped. With IE and Chrome they are fine.
Need this thing urgently sorted. But I have no idea how to solve that problem. Any help would be appreciated.

http://www.mzb-nord.de/produkte/depot-21/

Thanks for help.

Markus

http://wordpress.org/extend/plugins/nextgen-gallery/