Modular software can be pain - you end up installing tons of add-opns. I recently configured a new desktop box and had to reconfigure Firefox. Here's the lsit of extensions I installed, I'd be interested in comments about better or missing add-ins
Adblock Plus This is a tricky one. I get content for free for looking at some ads. Might be a fair deal, but often it's too much. On some sites I allow ads, on others not.
BetterPrivacy Prevent Flash Cookies from tracking me
Brief An RSS/Atom/... Feed reader
Certificate Patrol Notifies me when sites change their SSL certifactes, might prevent fraud.
Change Rerer Button I don't like being tracked. At least not always
CookieSafe Firefox's cookie handling is damn limited. With this add-in I can edit/delete single cookies way simpler and easily change the per site settings
Firebug Good for fixing broken sites
FoxyProxy Standard When switching between company VPN and my local net I need different proxy configuration. With this extension I don'T have to go through the settings
Greasemonkey Improve websites according to my interest
JSON View Renders json more nicely. Good when working with JSON-based protocols
Open in Browser Adds a new option to the download dialog to open in Browser as web page or plain text. Useful for files a server sends with a "wrong" header.
Tab Kit The wider the screen the more I want to have the tabs on the side. This add-in shows tabs in a sidebar with a tree structure and different colors and such
Tamper Data Edit HTTP Request before the browser sends them, initially I've got this for security research, nowadays I use it to work-around issues in bad WEb-UIs
User Agent Switcher Maybe switching the User agent every now and then makes tracking a bit harder, certainly it can be nice to get a different view. Many sites have a mobile version which is focussed on the content way mor than the regular site.
Web Developer Nice tool for working with HTML stuff