Firefox launched on November 9, 2004. It took 99 days to deliver 25 million downloads. In less than six months the number doubled, delivering 50 million downloads . At the end of the first year the 100 million downloads mark has been reached. One further year and it doubled to 200 million. One more year and it’s doubled again to 400 million downloads.

From the Mozilla blog:

Today, you all have done it once again. With your amazing efforts, Firefox has reached 400 million downloads and demonstrated that not even the world’s most powerful companies can keep people from a better, safer, and faster Web experience. You all, the grass roots and the heart of the Firefox movement, have helped hundreds of millions of people find that better, safer, and faster Web.

Thank you for building this movement. Thank you for helping Firefox to deliver on the great promises of the Web. On behalf of the hundreds of millions of Firefox users, thank you for all that you have done in just three short years.