In less than three years, Mozilla's Firefox has amassed 400 million downloads. The whole shift towards using the Firefox web browser was sparked in November 2004 by a Spread Firefox campaign in which tens of thousands of users spawned 25 million downloads. The buttons and banners placed on websites took only a year to grab 100 million downloads, and a year later 200 million.
http://www.downloadsquad.com/2007/09/08/firefox-hits-400-million-downloads/
This is quite an impressive feat. Popularity is growing constantly, so 500 million will happen in no time.
http://www.downloadsquad.com/2007/09/08/firefox-hits-400-million-downloads/
This is quite an impressive feat. Popularity is growing constantly, so 500 million will happen in no time.