Your gallery look similar in Firefox 19.0.2, IE10, Chrome 25 and Safari 5.1.7 on my PC.
Try clearing your browser's cache to ensure that it is not hanging onto and using older versions of your gallery files.
Please see this FAQ in case it describes your scenario and offers an explanation:
When I view my gallery I see a grid of thumbnails, not the layout I specified. Why?
Also, please see the following FAQ which is specific to Firefox in case it helps:
My gallery does not show in Firefox or gets cropped to a small height. Why?
A quick fix for the issue in the FAQ is to set your gallery's height as a fixed pixed value rather than as a percentage, e.g.:
simpleviewer.load("sv-container", "100%", "600", "FFFFFF", true);
However, as I said, your gallery looks OK in Firefox to me.
If you continue to experience difficulties, please explain what is wrong when you view your gallery in Firefox and, if possible, please upload a screenshot somewhere so that I can take a look and see what you are seeing. Thank you.
(When I validate your web page with the W3C Markup Validation Service, there are a few minor HTML errors which should be fixed but it looks like they are not likely to cause a problem with the display of your gallery.)
SimpleViewer Support Team