According to John Lilly, the Firefox web browser has 126 million unique monthly users. A figure extrapolated from an Active Daily Users number of 48 million, itself a massive increase on the 23 million ADU figure from this time last year. So who is John Lilly and why should we take any notice of his figures? Well, Lilly happens to be the Chief Operating Officer at Mozilla and the figures have been reached by analysing statistical data from the Firefox browser update service logs.

Lilly admits that the various browser market share reports, while having value are subject to different biases. "The most basic issue is that there's no way to really represent the complexity and the dynamism of the global Web - it's just too big, with too many things changing too rapidly" he says. So you get a global share reported at the top end as high as 36%, while Mozilla itself seems happy with a consolidated share garnered from numerous data reports at somewhere in the region of 20%. The problems of measurement are magnified by the metrics used, many of which happen to be in page views. "As hard as it is to get an accurate read on worldwide traffic numbers for Firefox" Lilly comments "it's even harder to figure out the relationship between page views and users. We have some intuitions here, supported by anecdotal evidence, that Firefox users look at more pages and do more searches than typical users, but nothing that I'd actually call science."

Which is why Mozilla has looked to its own systems to get a different measure on Firefox popularity globally, the number of active users out there. Firefox uses an application update service (AUS) system to jeep the software up-to-date with security patches etc. Every day, Firefox pings these AUS servers to check for any new updates. Don't worry privacy hounds, these pings are non-identifiable. However, Mozilla does count them, and categorize them by language version. This means that at any given time it is possible for Mozilla to know how many instances of Firefox are running around the planet. OK, so it's not going to be accurate to the last man or woman, but as a rough guide it's pretty darn good.

To arrive at the 126 million monthly users number, Lilly took the 42 million ADU figure and used the accepted website method of multiplying by 3 to convert it into monthly uniques. "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, piecing together data we've received from sites who have done their own calculations, and then really testing them against the best common sense top-down tests we can" Lilly insists.

I tend to agree with him, and think the actual figure is going to be much higher thanks mainly to the 'Firefox Dark Matter' phenomena that comes courtesy of Linux users. The problem being that users who get their Firefox fix as part of a Linux distro will most likely have the automatic update feature itself disabled, because the distro will manage those security updates instead. Lilley admits that this dark matter exists "but we don't have ways to see it. Linux use certainly accounts for at least single-digit millions every day (and maybe more), but it's hard to say exactly how many."

As Editorial Director and Managing Analyst with IT Security Thing I am putting more than two decades of consulting experience into providing opinionated insight regarding the security threat landscape for IT security professionals. As an Editorial Fellow with Dennis Publishing, I bring more than two decades of writing experience across the technology industry into publications such as Alphr, IT Pro and (in good old fashioned print) PC Pro. I also write for SC Magazine UK and Infosecurity, as well as The Times and Sunday Times newspapers. Along the way I have been honoured with a Technology Journalist of the Year award, and three Information Security Journalist of the Year awards. Most humbling, though, was the Enigma Award for 'lifetime contribution to IT security journalism' bestowed on me in 2011.

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