Html Validator: Validates Html and shows number of errors on an Html page.
IE Tab: Embeds Internet Explorer in tabs of firefox. An excellent tool to see how a web page looks in IE.
OperaView: Opens up a web page in the Opera browser. Good for quick cross-browser testing.
Total Validator: Performs Html validation, accessibility validation, broken link checking, spell checking and takes screenshots using 36 different browsers.
XSS Me: Find Cross-Site-Scripting (XSS) vulnerabilities in a web application.
HackBar: A security audit and penetration test tool.
Access Me: Tells you how vulnerable a website is to attackers and unauthinticated users.
SQL Inject Me: Find SQL injection vulnerabilities in a web application.
Molybdenum: Create, record, edit and run web tests easily.
Regular Expressions Tester: Allows developers to test regular expressions.
Firefox Accessibility Extension: Test your website for functional accessibility for people with disabilities.
CSS Validator: Validates a web page for CSS using the W3C rules.
WAVE Toolbar: Evaluate accessibility and usability of a website for people with disabilities.
TAW3 With A Click: Website accessibility testing based on W3C Web Content Accessibility Guidelines.
YSlow: Analyzes web pages to understand why they are slow. Based on Yahoo!’s Smush.it service.
HTML Regex Data Extractor: Test Regular Expressions on live HTML content.
Thanks to All