Firefox add-ons have been one of the biggest reason I and I believe several other users have still stuck with the browser. Firefox extensions or add-ons as they are known spice up the browser adding features which make it more useful and better.
Today, Firefox completed a huge milestone with over 2 billion downloads of various add-ons and Firefox themes. Firefox has reached the milestone with 2 billion downloads over a period of over 3 years from addons.mozilla.org. The below chart shows the downloads on a year to year basis.
Firefox had completed 1 billion downloads back in November 2008, the second million took over 19 months. The Firefox add-ons boasts of over 150 million add-ons in use every day. Mozilla has also created a special collection of add-ons called Best of 2 Billion Firefox add-ons which contains about 25 add-ons, you can view the list and download it from here. For more information on this milestone visit the Rock Your Firefox announcement.