Firebug Firefox Add-On

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.

Visit the Firebug website for documentation, screen shots, and discussion forums: http://getfirebug.com

Web Developer Toolbar for Firefox

This is a great add-on for Firefox.

The Web Developer extension adds a menu and a toolbar with various web developer tools.

You can validate your HTML or xHTML markup & CSS code, highlight certain objects, and many more useful functions.

Tamper Data Firefox Add-On

Extremely useful tool for testing/debugging web and database applications and form processing.

Use tamperdata to view and modify HTTP/HTTPS headers and post parameters.

Trace and time http response/requests.

Security test web applications by modifying POST parameters.