From the 'more fun with numbers' files:

The open source Firefox web browser from Mozilla now has some 270 million users. That's the figure that Mozilla staffer Aza Dotzler is now claiming, and it's not an easy number to calculate.

How do you count users for an open source application that does not require registrations? Downloads aren't accurate, since downloads include the same users that could have updated and/or downloaded revisions.

What Mozilla does is they use their Application Update Service (AUS) -- the service whereby Firefox automatically calls home once a day looking for updates--  as a method. The number of users that hit AUS on a daily basis are called the Active Daily Users. Dotzler notes that there are now 90 million active daily users. As part of a mathematical computation, Mozilla multiplies the number of daily users by 3 (hence the 270 million) to try and gauge how many total users they have -- since not all user are active every day (or so the theorem goes).

Personally, I like the way that Fedora Linux tracks its user numbers which is a similiar approach. They also measure how manys servers 'call home' looking for updates, though Fedora adds in the measure of checking by unique IP address (which presents its own issues).

Talking about Linux, Mozilla's numbers for Firefox do not include the majority of Linux users. That's because Linux users typically do not get Firefox from Mozilla directly but rather by way of their own distributions update repositories (so they don't check Mozilla's AUS).

If we take Fedora's number of users to be 13 million, Ubuntu Linux's to be another 8 million, then roll in Debian, SUSE, Mandriva and Gentoo users, I don't think it would be unfair to say that Firefox easily has over 100 million active daily users and nearly 300 million users total (but that's my own calcuation).

No matter how you slice it, Firefox is likely the most popular open source application in history (in terms of usage) and it continues to grow.