The number of Firefox users has doubled over the last year or so, according to a post on the blog of Mozilla COO John Lilly. Lilly says there are now "at least" 125 million users running Firefox worldwide. He also mentions that the average number of active daily users has gone up from 23 million to 42 million since October 2006.
Mozilla obtained those figures by harnessing Firefox's automatic update mechanism. Since Firefox checks for updates once a day, Mozilla averages the number of pings from unique users over a seven-day period to get an Active Daily Users (ADU) figure. The 125 million number is simply the ADU multiplied by three.
Lilly explains: "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." That number is actually more reliable than market share estimates, he says, because 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 rapdily." (Thanks to Ars Technica for the tip.)