Here’s a cool plugin for Firefox which allows you to debug Ajax code.

FireBug is a new tool that aids with debugging Javascript, DHTML, and Ajax. It is like a combination of the Javascript Console, DOM Inspector, and a command line Javascript interpreter.

Other fun features:

1. * XMLHttpRequest Spy - Ever wonder what all them newfangled Ajax websites are up to? Watch the requests fly by in the console!
2. * One web page, one console - Tired of slogging through a zillion errors in the JavaScript Console trying to find the one you want? The FireBug console is built into the bottom of the browser, and only shows you errors and log messages that came from the page you’re looking at.
3. * JavaScript Error Status Bar Indicator - It’s a sin that Firefox doesn’t include this by default, like IE does. When there is an error in the page, the status bar will let you know with a big red blob.
4. Logging for web pages - Sick and tired of “alert debugging”? Jealous of all your C programmer buddies with their fancy printf? Now you can log text and objects to the FireBug console from any web page.

Link to Plugin Page:Mozilla Update :: Extensions — More Info:FireBug - All Releases