Found a cool feature in Firebug in that you can easily debug response codes of ajax requests made from the actual javascript (rather than hitting the ajax all page directly).

Hover over ‘Net’ in the top menu, then select ‘Enabled’. You will then see the submenu containing ‘XHR’, click it. The area below the submenu will now show you response codes from any embedded ajax requests.

Firefox /w Firebug - Ajax Debugging

Firefox /w Firebug - Ajax Debugging