Borders causing DIV to drop?

Hello there.

Having a slight problem with a page I’m trying to build.

I noticed when I zoom out in firefox ( using ctrl+ - ) the far right DIV drops.
This only happens when I have a 1 pixel border around each box. Without the borders it works as expected and the DIV does not drop.

  • When I apply the 1 pixel border I subtract 2 pixels from the width of each box. Altogether they should come to 960px - the width of the containing DIV.

I’ve uploaded the page ( with CSS in header ) to:
http://www.ok-hybrid.com/store/argh.html

Any help, much appreciated.