A study of "pings" to Mozilla's servers estimates nearly 49 million may be using a version of Firefox on a daily basis. That's 15 million more than a Mozilla study estimated, using a different metric, for users in July.

It has been notoriously difficult to determine how many people use the Firefox browser, though many have come to an agreement that the number of downloads of any new version is not a reliable indicator. Today, a new estimate by Mozilla's Chief Technology Officer, John Lilly, makes a claim that would appear to be a significant adjustment to one just four months ago by Mozilla's user experience lead, Mike Beltzner, by way of some unique mathematics.

As of last week, the CTO wrote on his corporate blog, the number of pings sent to Mozilla's update servers by installed clients in the field registered a record of 48.8 million. Each of those pings, Lilly wrote, represents a single daily user.

To convert that figure into the total number of worldwide users per month, he states, all you have to do is take that figure and multiply it by 3.

"To get from the [Average Daily User] number to our whole worldwide number of users, measured in terms of uniques in a given month, like most every Web site does," Lilly writes, "we multiply ADU by 3. So for a couple of weeks ago, with 42 M ADUs, we compute that we have something in excess of 126 M unique monthly users."

Just last July, however, Mozilla's Beltzner made what appeared at that time to be an equally astounding claim: 100 million unique monthly users for Firefox, based on an estimate of active daily users that was closer to 33 million - about one-third lower than Lilly's record of last week.

But helping to substantiate Lilly's more recent claim is a separate blog post yesterday by Mozilla's analytics chief, Ken Kovash, who presents a formula that some may use to arrive at the 126 million total user mark from the opposite direction: Citing a formula being tested by what Mozilla calls "Project Funnel Cake" -- a way of assessing likely usage share based on reliable figures -- Kovash estimates that the fraction of Firefox downloaders who actually install the product on their system is a surprising 57%, and that the number of the remainder who go on to use the product on a daily basis is 49%.

As of today, a check of the SpreadFirefox.com live counter estimates over 448.7 million total Firefox downloads, life to date. Applying Kovash's formula to this number, one arrives at 125.3 million -- very much in line with the figure Lilly arrives at by multiplying AUS pings by 3.

"Of users downloading Firefox, 28% become active daily users," writes Kovash. "That sounds like a pretty good retention rate to me. The Firefox community deserves a great deal of congratulations for this type of success."