YSlow analyzes web pages and suggests ways to improve their performance based on a set of rules for high performance web pages.
http://yslow.org is the official open source website.
This extension provides advanced configuration for your browser. It allows you to easily change many additional options, which are not displayed in the Firefox Options dialog.
CLEO (Compact Library Extension Organizer) is a Firefox extension that works with FEBE* to package any number of extensions/themes into a single, installable .xpi file.
Launchy will enable you to open links and mailto's with external applications...
The about:me page provides a fun way to see personalized patterns and trends in browsing history.