Firefox Page Margins Off

Hi I have a small problem with my web site, I believe the solution to be an adjustment in the style sheet. I am using Zen vs 1.3.0.2 the problem only occurs in Firefox and is ok in IE.
The side boxes on the right move out of the centre white part to the right. For example purposes please see http://www.colourshield.com/store. When I remove the navigation bar the problem vanishes so I know thats pushing the limits on the page margins, however thats as small as I can make the nav bar, my solution I thought of was to increase the white area by a few pixels and see if that fixes it. However I am not sure where the adjustment is for this or if it is the problem.
Any Help would be greatly appreciated, By the way how does the rest of the site look..I have just revamped it.
Thanks and Regards
Renz

I pulled your site up in firefox twice and it came up fine in one instance and showed the right column problem you mentioned in the second.

stylesheet_original.css and stylesheet_new.css are both being used. Is something in one canceling out the other? I think that is your problem plus you really need to update to the latest version 1.3.8 if just for the updates on security.

I did notice your contact page is not secured.