Firefox has 125 million users

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.