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The figure comes courtesy of John Lilly, Mozilla's Chief Operating Officer, who arrives at the result by analysing the statistical data collected through the browser's update service.

As explained on Lilly's blog, Firefox pings the update servers every day to check for patches and new versions, and by counting

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those pings he says it is possible for Mozilla to make a rough guess at how many instances of the browser are running.

He labels these Active Daily Users (ADU), and says that taken as an average over seven days, the number of ADUs has skyrocketed from 23 million in October 2006 to 48 million in November.

"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 website does, we multiply ADU by 3," Lilly explains in his blog.

"So for a couple of weeks ago, with 42M ADUs, we compute that we have something in excess of 126M unique monthly users."

Intriguingly, whether you agree with the maths or not, Lilly says he actually considers the multiplier to be conservative.

"Take our estimate of monthly users (126M) and divide by the whole number of internet users in the world (Internet World stats puts the current [population] at 1.2 billion) - you end up with about 10.5%, which is lower than most reports of our global traffic share."