From the department of the-best-things-in-life-are-free:
"Firefox hit another milestone this past Friday, when it passed the 400 million download mark. From its launch in 2004 it took one year to reach 100 million downloads, hitting 200 million downloads just one year later. According to figures released by US consultancy firm Janco and the IT Productivity Center, Firefox currently has 17.4 percent of the browser market — up 5.6 percentage points in the last year. Also within the last year, Microsoft's Internet Explorer browser dropped 9.6 percentage points to a market share of 63.9 percent."