I've been using Firefox for a long time, and have always had Web developer plugin by my side for those miserable days. This tool which can save your ass at a time when you really need to understand what the heck your Ajax code is up to.
A couple of days ago I came across another such tool calledÃ‚Â Firebug. All I have to say is that I was completely blown away by its intutive debugging style Cleaning up my messy Ajax generated code could have been a lot worse if this guy wasn't around.
Here is a quick feature list
* Status bar icon shows you when there is an error in a web page
* Spy on XMLHttpRequest traffic
* Inspect HTML source, computed style, events, layout and the DOM