Customize Firefox themes – Provides a large collection of new themes for changing your Firefox look and feel.

Mozilla’s Official Way to Customize Firefox – shows undocumented features and ways to customize their browser.

How to Use GreaseMonkey – A must-read if you truly want to take your browsing to the next level.

Office Firefox Add-on Library – Contains an amazing amount of add-ons that you can install with a simple click.

Mashable’s List of 30+ Firefox Extensions – Extensions that enhance your browsing experience.