A major problem with developing AJAX applications is trying to follow data flowing back and forth between the server and client and the changes being made to the DOM.
Enter FireDiff to ease the pain! Firediff is an extension for Firebug that adds a change monitor, logging all changes made to the CSS and DOM. Which makes it a must for most web developers, FireDiff requires Firebug 1.4 or higher to work, and you can find the latest version to install from http://www.incaseofstairs.com/download/firediff/