Firebug

Firebug integrates with Firefox to put a wealth of development tools at your fingertips while you browse. You can edit, debug, and monitor CSS, HTML, and JavaScript live in any web page. Firebug gives you full control over the CSS, HTML and JavaScript of any page that you choose. Inspect and edit HTML, Tweak CSS to perfection, Visualize CSS metrics, Monitor network activity, Debug and profile JavaScript, Quickly find errors, Explore the DOM, Execute JavaScript on the fly, Logging for JavaScript. Best of all, every change that you make is done live and instantly, so you can quickly test out different changes and see the results of those changes instantly.