I keep getting asked what FF extensions Ii use well this is what i have installed at them moment :

User Agent Switcher Extension
http://www.chrispederick.com/work/firefox/useragentswitcher/
This extension adds a menu to switch the user agent of the browser. So you can even pretend to be Googlebot ï?Š

Web Developer Extension
http://www.chrispederick.com/work/firefox/webdeveloper/
This is a must have for web devs, you can just about doing anything with it, disables on screen elements , edit css on the fly

Gmail Plugin
http://www.gmailwiki.com/index.php/Tools_and_Utilities
adds little icon at the bottom of the screen and pops up when you have Gmail

Spellbound Plugin
http://spellbound.sourceforge.net/
spell checker a must if you speeling is reilly bad

IE view Plugin
http://ieview.mozdev.org/
It allows the current page or a selected link to be opened in Internet Explorer

Create Image tags
http://kostecki.net/fb/
one click and it creates the code to add an image

colourzilla
http://www.iosart.com/firefox/colorzilla/
lets you choose the colour off a webpage

Page Downloader
http://downthemall.mozdev.org/
can be set to exclude all sorts of fun

Ad Blocker
http://adblock.mozdev.org/
block those ad servers

HTML VALIDATOR (based on Tidy)
http://users.skynet.be/mgueury/mozilla/
I use this all the time when viewing other people source code

About this site
http://www.scribbling.net/projects/firefoxextensions/aboutsite/
gives you info on the page you are looking at backlinks, whois, alexa etc

DaveN