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:
Download Firefox 2.
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.
