Firefox extensions are a way to enhance your surfing experiences. Many people just don’t know that they exist or just forget about them. Here a list of the more powerful and useful extensions:
Google Notebook (Highly recommended) from Google
ScrapBook (Highly recommended) I can’t live without it!!
Tab Mix Plus
User Agent Switcher
More in http://www.mozilla.com/add-ons/
And a screenshot of my installed extensions: