You'll need to do some fiddling with margin and padding (and maybe height) in various places in your stylesheet. Install Firefox and the Web Developer plugin so you can see what's what.

The red bars, which are #navMain (mostly) look fine in Firefox 2.

The issues in the product info pages are also margin/padding things, and Web Developer will help out there, too.

'Store Front' is a link - it links to your store's home page, which happens to be blank.

https://www.zen-cart.com/tutorials/index.php?article=82