Visit the FireBug website for documentation, screen shots, and discussion forums:
A quick overview of FireBug's features:
* 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
Firefox 1.5 - 3.0 ALL
I was very frustrated doing AJAX/AHAH development until I found this tool. Firebug lets me inspect any part of the DOM I want, and it lets me look at the XMLHttpRequest and its response. This, along with Catalyst's DBIx::Class debug information from the console allows me to monitor my web application from the client presentation to network communications to server side processing to database queries and back. Pretty damned cool.