Firebug - Web Development Evolved.

Firebug integrates with Firefox to put a wealth of web development tools at your fingertips while you browse. You can edit, debug, and monitor CSS, HTML, and JavaScript live in any web page. Its features include:

• 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