Here is the problem with your categories. The width of each category box is 159px, the total width of the 3 categories box is 477px and the total width of the center box is 473px. So you have additional 4px that pushes the site to the second page.

I was able to fix that in the CSS. Open your CSS file around line 150 look for .categoryListBoxContents you will notice that there is padding of .4em, You can remove the .categoryListBoxContents from the list or copy it on the second line and adjust the padding to the right width.

Hope that fixes your issue. I tested in FF.