There are tons of great firefox extensions to choose from – this post does not attempt to catalog them all.
This setup does everything I need as an SEO and Web Developer, is lite, and rarely (if ever) crashes:
Web Developer – Hands down the most useful Firefox Plugin Ever.
SearchStatus – Display the Google PageRank, Alexa rank and Compete ranking anywhere in your browser, along with fast keyword density analyser, keyword/nofollow highlighting, backward/related links, Alexa info and more.
ShowIp Show the IP address of the current page in the status bar. It also allows querying custom services by IP (right mouse button) and Hostname (left mouse button), like whois, netcraft. Additionally you can copy the IP address to the clipboard.
Dom Inspector – Inspect the DOM of HTML, XUL, and XML pages, including the mail.
Downloadhelper – The easy way to download Web videos from hundreds of YouTube-like sites. Also works for audio and picture galleries.
Youtorrent Search Plugin – THE best way to search for torrents: Search all the torrent sites at ones.
Custom US Google Search – Click the “OpenSearch plug-in Google USGoogle Toolbar” link near the bottom, after the “Change the plugin”-button. If you’d want more options with different localization, just change the ‘gl’ parameter in the url to be the country code for the localization you want.
Via File -> Preferences, I set homepage(s) to:
http://popurls.com/|http://mail.google.com|http://www.seoblackhat.com/forums/|http://bloglines.com/myblogs
The | allows for multiple tabs to be opened on launch.
Add a couple of bookmarks to the quicklaunch bar and we’re done.

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April 9th, 2008
QuadsZilla
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Also I’d add:
* Colorzilla
* CustomizeGoogle
* LiveHTTPHeaders
* TableTools
* User Agent Switcher
About This Site is a good extension also.
For me Firebug is a must have. I discovered it two weeks ago while working on a Drupal installation and it was indispensable.
Throw out Dom Inspector and Web Developer for Firebug.
Firebug is the best extension for web development, hands down.
SearchStatus works!
But FireBug never works
I personally prefer SEOQuake over SearchStatus:
http://www.seoquake.com/
Nifty things include:
Ext Inbound Links pointing to current page & entire Domain [Yahoo! Data]
Amount of indexed pages on G, Y & M
Links from Technorati, Digg, Del. etc…
IP Address & WhoIs lookup
Robot.txt Yes/No & easy show
GPR
& SEODigger.com which is good for getting quick snatch data on competitors ranking keywords.
Oh and great post btw. I’m still totally shocked at how many of my competitors when I meet up with them at conferences etc, have never heard of FireFox addons. Gah…. People ACTUALLY crawl/query data in such old fashioned ways still to this very date. Shocking. I shall pass the post on!