One of the greatest things about the Firefox browser is that any developer can expand its features and capabilities with a simple add-on. There are add-ons for everything—Twitter, Facebook, Stumbleupon, Digg, feed readers, price comparison, photos, music, and more. Today, every browser supports plug-ins and extensions.

Yesterday, cumulative add-on downloads passed the 2 billion mark. It passed the first billion downloads back in November, 2008. The first billion took three and a half years. The second billion took about 20 months. Many of those are never used again, but a full 150 million add-ons are used every single day at least once.
TechCrunch