FireFTP
Stop looking for FTP clients, you got Firefox baby :8)
FireFTP is a free, secure, cross-platform FTP client for Mozilla Firefox which provides easy and intuitive access to FTP servers.
https://addons.mozilla.org/en-US/firefox/addon/684

Firebug
No more Notepad for quick editing
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
https://addons.mozilla.org/en-US/firefox/addon/1843

IE Tab
Testing made Easy
IE Tab - an extension from Taiwan, features: Embedding Internet Explorer in tabs of Mozilla/Firefox.
https://addons.mozilla.org/en-US/firefox/addon/1419

ColorZilla
Steal that color, Stop pressing Print Screen
Advanced Eyedropper, ColorPicker, Page Zoomer and other colorful goodies.
https://addons.mozilla.org/en-US/firefox/addon/271

YSlow
Get answered by Yahoo!
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

SeoQuake
Analyze That! :8)
Seoquake allows to obtain and investigate many important SEO parameters of the internet.
https://addons.mozilla.org/en-US/firefox/addon/3036

Domain Details
Quick Check!
Displays Server Type, IP Address with Location Flag, and links to various Whois Reports
https://addons.mozilla.org/en-US/firefox/addon/2166

Page validator
Validate your Wordpress!
Validates a page using the W3C Markup Validation Service.
https://addons.mozilla.org/en-US/firefox/addon/2250

RankQuest SEO Toolbar
All in one!
RankQuest SEO(Search Engine Optimization) Toolbar provides you quick access to more than 30 intuitive SEO tools. Alexa Rank and Page Rank provided by Alexa and Google respectively ensures the popularity of the site.
https://addons.mozilla.org/en-US/firefox/addon/1471

Google Global V2
Get Specific! :8)
View country specific search results on google.com ( + a lot more in V2, Follow the link )
http://www.redflymarketing.com/blog/google-global-view-results-different-locations/