Extensions are an extremely important and powerful part of Firefox. Extensions add features and functionality to Firefox that is not available in other browsers. Extensions are written by independent developers wishing to add a feature they find useful to the browsers. These tools can be utilized by power users to make sweeping changes to Firefox and make it perform functions well outside of what a normal browser without extensions is capable of. Some of the most common extensions are Adblock (which give the option to black any ads on a webpage), All-in-One Gestures (control the browser with mouse gestures), Image Zoom (can zoom in on an image), Tab Browser Preferences (improves tabbed browsing), and ForecastFox (displays weather information from the Weather Channel).
To find extensions go to http://updates.mozilla.org
Click on Extensions near the Firefox icon.
From this page you can browse the extensions or search for a specific extension. Once you find the extension you are looking for go to its page and click Install Now.
The following screen will appear, click Install Now.
The extensions window will come up and show a message that the extension will be installed once you restart Firefox. Close Firefox and open it up again and the extension should be functional.