The Mozilla Foundation's Firefox Web browser passed a significant milestone in adoption on Friday, with more than 50 million copies of the program downloaded, according to its distributors.

To commemorate the moment, the foundation said on its Web site that it would create 50 limited-edition coins, to be distributed to people with stories of spreading the browser online. An additional, a still-unnamed prize will be given to the owner of the Web site responsible for the 50 millionth download. "It's funny how the counter just blows by 50 million without a care in the world, isn't it?" Mozilla developer Blake Ross wrote on the foundation's Web site. "But it's not just a number to us. It's a validation of half a decade of work, and the beginning of half a decade more." With its first full-fledged release last November, Firefox has shaken up a Web browser market that most analysts had deemed almost wholly mature. For the first time in years, the market share of Microsoft's Internet Explorer has begun inching downwards as Firefox adoption rises.