Within Chrome, it's possible to place a breakpoint on a XmlHttpRequest allowing you to debug AJAX requests. You can add a breakpoint by going to the "Sources" tab and selecting "XHR Breakpoints". Click the plus icon and type part or all of the URL you wish to add a breakpoint for. The breakpoint will occur once the request has been returned, allowing you to find out what data has been returned and step through what happens next.

The breakpoint can be turned off by unchecking the box, or deleted by right clicking and selecting "Delete Breakpoint".

Firebug allows breakpoints on XHRs by selecting the "break-on-next" button in the Network tab to break on the next one, or by right-clicking and choosing "Break on XHR". You can optionally choose a breakpoint condition as well.