Extensions are one of the major selling points for Firefox. No other browser can match the sheer volume of third-party mini applications and enhancements.
Most Firefox users are aware of the concept of extensions, but not as many people know that Extensions (and Themes) are available for its email client cousin Thunderbird as well.
First of all, head over to the Thunderbird Add-ons site in your web browser. It is almost exactly the same as the Firefox Add-ons site and works in largely the same way.