Computerworld is running a story on the 20 must-have Firefox extensions.

I'm not a regular Firefox user (I use OmniWeb), but I do find Firefox is by far the best browser to use when developing web pages through tools like http://livehttpheaders.mozdev.org/installation.html, which is invaluable when you are diagnosing cookie issues.

But there are more - GreasMonkey rocks, and there are plenty of tools that will pretty much completely change Firefox from a browser into somethign more. This trend for a ruch basic platform plus lots of extensions (plugins) is becoming increasingly popular.