Mozilla COO John Lilly has revealed statistical data that provides insight into Firefox adoption levels and growth rate. Based on statistics collected internally by Mozilla from the Firefox application update service, Lilly estimates that Firefox is used by over 125 million people around the world.

Firefox market share statistics often attract the interest of technology enthusiasts and market analysts, but they don't always paint a clear picture. Many Firefox market share studies are based on traffic data collected from a broad selection of sites. Lilly comments that studies of that nature can present skewed results depending on the kinds of sites that are incorporated into the research. Sites that are geared towards technology enthusiasts and early adopters, for instance, often have a higher percentage of Firefox users. Lilly also notes that European web sites tend to see more Firefox traffic than sites from the United States, and virtually all of the studies, he argues, undercount browser usage in Asia, South America, and Africa.

"[T]here'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," says Lilly. "[A]ll of these studies are extremely useful and help understand what's happening around the world. I'm just asserting that it's very important to understand the limits of particular studies and the assumptions that are baked in."

Lilly also points out that most of the browser market share statistics available from third parties are based mainly on measuring page views rather than users, which he thinks could potentially overstate the extent of Firefox market share. "[M]ost of these measures are in page views, not users," says Lilly in reference to oft-cited third-party browser statistics. "As hard as it is to get an accurate read on worldwide traffic numbers for Firefox, 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."

Lilly arrives at the 125 million user estimate by leveraging the statistical data collected by the Firefox application update service. The Firefox web browser sends an anonymous request to the update servers every single day to check for new versions. Counting the total number of pings makes it possible for Mozilla to guess roughly how many instances of Firefox are running on any given day, a metric that Mozilla refers to as Active Daily Users (ADU). This number has climbed from 23 million in October 2006 to 48 million at the high point in November, Lilly says.

The ADU number obviously doesn't come close to accounting for the total number of Firefox users, but it serves as the basis for the computation. There are many other factors in play here. Not every Firefox user is using Firefox every single day. Proxy usage and corporate firewalls could also potentially be preventing a large number of running Firefox instances from communicating with the update server. It's also worth keeping in mind that the vast majority of Linux distributions ship Firefox with the browser's built-in update mechanism disabled because the browser gets updated by the distribution's package manager.

This is where Mozilla's math starts to get fuzzy. To arrive at the 125 million user estimate, Lilly multiplies the ADU by 3. "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," says Lilly.

One of the tests Lilly describes for evaluating the accuracy of the multiplier is to divide the estimate by the total number of Internet users in the world and see how that compares to standard market share statistics. Lilly points out that when you divide 125 million by 1.2 billion (the total number of Internet users as estimated by Internet World Stats) you get roughly 10.5 percent, which is actually lower than the global market share attributed to Firefox by most third-party studies. With that in mind, it seems safe to assume that there are at least 125 million Firefox users.

Although I don't think that Mozilla's computations based on ADU are significantly more accurate than other browser market share studies, the ADU statistics provide unique insight into several aspects of Firefox usage patterns. The fact that the ADU has doubled since the release of Firefox 2 is a sure sign that Firefox market share is growing rapidly. Although Microsoft is making a strong effort to regain market share, we can probably expect to see a surge in favor of Firefox when Firefox 3 is released next year.