FireBug: Web Debugging Made Easy in Firefox
Debugging is probably one of a developer's least favorite tasks but, regrettably, it's an essential part of software development. Any helping hand is therefore received with open arms and FireBug does much of the hard work for you. FireBug is a Firefox extension that allows you to monitor and tinker with JavaScript, CSS, HTML, and Ajax through a combination of a powerful debugger, error console, command line, and code inspectors.
The debugger sets breakpoints in your scripts and examines every step of execution in real time. Particularly handy is the FireBug icon in the Firefox status bar which tells you instantly if your page is broken. Just open the FireBug panel and you'll see exactly what the problem is - no more scouring through reams of script in the JavaScript Console window. You can open Firebug in a separate window, or as a bar at the bottom of your browser, and also configure a blacklist of sites that you'd like it to exclude.
For any developer who has been driven mad by debugging, FireBug provides an ultra quick, easy, and headache-free way of developing your code.