Mozilla Firefox gains in popularity and is still the strongest competitor for the Internet Explorer. Recently Mozilla Firefox crossed the 300 million downloads mark without much fanfare.
I remember the grand celebrations when Firefox crossed 50 million (April 2005), then 100 million (October 2005) and then 200 million (August 2006). You can easily keep track of the Firefox download count by adding a Firefox download counter to your site.
It is important to realize that this is not the actual number of users since the multiple times you upgrade Firefox are also counted. Cybernet has plotted a graph to track the Firefox download milestones and predicts 400 million downloads will occur August 25, 2007 and 1 billion downloads by November 3, 2010 and the list goes on till a trillion.
Compare that with of Microsoft Internet Explorer 7 clocking 100 million IE7 installations within 3 months of launch.