I have been a long time user of Firefox. In fact, I have been using it since it was named Phoenix. One of the best features of Firefox is it's modular support for extensions. I have used so many extensions over the past few months, but here is a list of the top ones that I use daily:
All-in-One Gestures - You just can't be without mouse gestures
BugMeNot - Fills in login forms for websites that require free registration to read a single news story
ColorZilla - Great for developers. It allows you to sample colors out of your browser window and copy their RGB or hex color code
CuteMenus 2 - Adds nice icons in all the menus
Disable Targets For Downloads - Prevents download links from opening new blank windows/tabs
Download Manager Tweak - Heavy mod to the download manager. A must have.
Fasterfox - Performance and network tweaks for Firefox.
FirefoxView - Right click in IE and select "View in Firefox" to open that page in Firefox.
Forecastfox - Weather forecasts right in your status bar
Gmail Notifier - What else?
IE View - Right click in Firefox and select "View in IE" to open that page in IE.
ListZilla - Allows you to export theme and extension lists to HTML or Text files. Great for ensuring you got all your favorite extensions.
Minimize to Tray - Self explanatory. Probably my favorite extension.
Plain Text to Link - Converts plain text urls to Links
SmoothWheel - Smooth document scrolling
Tabbrowser Preferences - Tab management
Talkback - Crash reports to Mozilla
View Rendered Source Chart - Color coded source code chart. Makes traversing a page's source much easier
Web Developer - Incredible toolbar. A must have for web developers.
Outlook 2003 Blue - My primary Firefox/Thunderbird theme. (Also available in Green and Silver)
ifox - My backup theme