Fireshot:
Use Fireshot to take screenshots of some of your web pages.
https://addons.mozilla.org/en-US/firefox/addon/5648
Pro version:
http://screenshot-program.com/fireshot/demo.php
Web Developer Tool:
Adds a menu and a toolbar with various web developer tools.
https://addons.mozilla.org/en-US/firefox/browse/type:1/cat:4
YSlow:
YSlow analyzes web pages and tells you why they’re slow based on Yahoo’s rules for high performance web sites.
https://addons.mozilla.org/en-US/firefox/addon/5369
SQL Injection tool:
Test and identify SQL injection vulnerabilities in your web pages.
https://addons.mozilla.org/en-US/firefox/addon/6727
Selenium:
Selenium is a Firefox extension that allows you to record, edit, and debug tests.
http://selenium.seleniumhq.org/
W3C Page Validator:
This tool will help you to validate a page using the W3C Markup Validation Service.
https://addons.mozilla.org/en-US/firefox/addon/2250
iMacros:
This tool will help you to automate some tasks on the web for activities you do repeatedly.
https://addons.mozilla.org/en-US/firefox/addon/3863
Pencil:
https://addons.mozilla.org/en-US/firefox/addon/8487
Firebug:
https://addons.mozilla.org/en-US/firefox/addon/1843
Source: https://addons.mozilla.org/en-US/firefox/