It looks like the cart display has been modified from its default. When I add items to the cart on your site it is not forcing the new item to be added on the next row, also the alignment in the cart is off.
I'm including a screen shot of what I saw in FireFox 3 after adding a few items to the cart. The regular PowerStore cart page does not display in this way. If you have altered the cart display on the page this may be the reason you are seeing these results. Also, I noticed a menu on the left side, it may be possible that the inclusion of this menu is causing some problems. At what point did the display change from it's default behavior?