NoScript Firefox Add-On

The best security you can get in a web browser!
Allow active content to run only from sites you trust, and protect yourself against XSS and Clickjacking attacks.

Winner of the “2006 PC World World Class Award”, this tool provides extra protection to your Firefox.
It allows JavaScript, Java and other executable content to run only from trusted domains of your choice, e.g. your home-banking web site, guarding your “trust boundaries” against cross-site scripting attacks (XSS) and Clickjacking attempts, thanks to its unique ClearClick technology.
Such a preemptive approach prevents exploitation of security vulnerabilities (known and even unknown!) with no loss of functionality…
Experts do agree: Firefox is really safer with NoScript

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

ColorZilla Firefox Add-On

This add-on for Firefox is a must have if you’re a graphic, print or digital artist, web developer, programmer or anyone that uses RGB, hexidecimal, or CMYK color codes on a daily basis.

Add-On is extremely light-weight and unobtrusive and features:

Advanced Eyedropper, ColorPicker, Page Zoomer and other colorful goodies.

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.