Firebug won't help in this case. The error is not in your browser, but on the server hosting ZenCart. You need to look at the error logs on your server, probably via the control panel on your hosting service.
Have you read mc12345678's post above where he mentions the fpdf.php problem and links to this post?
http://www.zen-cart.com/showthread.p...32#post1218332
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