While Microsoft published the latest data on the adoption of Internet Explorer 7, claiming an excess of 300 million users worldwide, Mozilla has some numbers of its own about Firefox. Mozilla COO, John Lilly, made public the statistics on the open source browser, claiming a market share of over 125 million users. Lilly stressed the fact that the 125 million users was only an approximation, but still indicated that Firefox has doubled its market share in a single year, benefiting from the release of Firefox 2.0.

"Lots of people report market share for Firefox - it's a succinct indicator of momentum in the market and with users, and makes a great headline to report that we're eating IE's share worldwide, or have exceeded 30% in Germany, or have tripled our share in China this year. And by any account, our market share everywhere is growing - extremely quickly in places like Spain and China and Brazil, somewhat more moderately in the US and Japan - but growing in every locale around the world," Lilly stated.

However, while Lilly was hesitant in giving the actual number of Firefox users, and has instead presented the statistics as an approximation, the fact of the matter is that, unlike third-party market share reports, Mozilla has access to the data from the Application Update Service. Mozilla simply counted all the pings coming from Firefox instances in AUS.

"Here's how we get to that number. Firefox uses a system that we call AUS (Application Update Service) to keep itself up-to-date with security patches & such. Around once a day, Firefox will ping Mozilla servers to see if there's a new update available, and if there is, it'll present an option to users to download and install it," Lilly explained.

At the end of November, Mozilla had some 48.8 million Active Daily Users (successful pings). This number simply gets multiplied by 3, in order to get an estimatation of the actual number of users. "To get from the ADU number to our whole worldwide number of users, measured in terms of uniques in a given month, like most every web site does, we multiply ADU by 3. So for a couple of weeks ago, with 42M ADUs, we compute that we have something in excess of 126M unique monthly users. This is a conservative multiplier (we think it could be more like 3.5) that we've gotten to by doing some of our own experiments," Lilly said.