Necessary AJAX developers tool Rated 5 out of 5 stars
FirePHP is extremely useful tool for error tracking and controlling your code flow as needed.
I found it irreplaceable for debugging server response in hidden iframes as well as with xmlHTTPResponse.
This new feature brings us simple and powerful solution, really easy to use either you decide to use oop coding style or if you prefer to write your code in procedural fashion.
I recommend adding FirePHP to your php include path if you intend to use it on multiple projects, but you don't have to.