Christoph Dorn has posted a guide to integrating the FirePHP tool into your application to aid in Ajax debugging.

My purpose in this column is to introduce you to FirePHP, the problem it solves, and how it is intended to work within your application for maximum benefit. I will present some useful high-level knowledge to make it easier for you to integrate FirePHP into your application. This is not a step-by-step tutorial. I have made a complete demo available for download, along with some useful libraries illustrating all the concepts covered here. I want to emphasize that the demo is only one possible implementation. There are many other possibilities.

He sets up his problem - figuring out a good way to debug Ajax applications without resorting to alert boxes - and the solution he found worked best for him. By using the FirePHP library he could send back the results (or any other information) from the backend call without needing to go through any more hassle than installing Firebug and the FirePHP tool. (Obviously, this only works in Firefox. Internet Explorer users might want to try out Fiddler.)