Firefox is beating Google's Chrome where it counts, in the number of users running each browser, a Mozilla executive argued Thursday.

Asa Dotzler, Mozilla's director of community development, made the case that Firefox's growth is not lagging behind Chrome's, as data from multiple Web metrics companies has indicated for the last six months.

According to measurement vendors like California-based NetApplications, Chrome's browser usage share gains now regularly outpace those of Firefox. In April, NetApplications' data had Chrome adding 0.6 of a percentage point of share while Firefox added only 0.07 of a percentage point, or just over an eighth as much.

Over the last 12 months, Chrome added 4.8 percentage points of usage share by NetApplications' reckoning; Firefox added 0.8 of a percentage point in the same period, again one-eighth as much.

Dotzler sees it differently.

Rather than measure usage share - which in NetApplications' case is based on a month's unique visitors to the sites it monitors for clients - Dotzler argued that a more accurate metric is the number of "active" users.

Mozilla measures such users by tracking the number of Firefox browsers that are launched each day. The company can come up with the number of running copies of Firefox because the browser pings Mozilla's servers to see if a newer version, such as a security update, is available, Dotzler confirmed today. Google monitors the number of active Chrome users in a similar way.

"In Firefox's early days, as in Chrome's early days, most of our early adopters were 'power users' who browsed a lot, dozens, maybe hundreds, or thousands of pages/sites a day," said Dotzler on his blog. "But today we have many more mainstream users than early adopter power users. Regular folks don't browse that much."

In other words, it takes many more mainstream users - "scores, or even hundreds," claimed Dotzler - to equal one power user. From Dotzler's chair, that means a relatively small number of active Chrome users produces an inflated browser usage share.

Dotzler said that Firefox currently has more than 360 million active users, while Chrome - by Google's admission earlier this week - has just over 70 million.

More importantly, Dotzler claimed that since July 2009, Firefox added over 100 million active users, or 2.5 times the 40 million additional active users Chrome gained in that same period.

Not surprisingly, Vince Vizzaccaro of NetApplications disagreed with Dotzler's painting of usage share numbers as inaccurate. "We weed out page views, and just count unique daily visitors," said Vizzaccaro, referring to the 160 million unique visitors the metrics company uses to come up with its share figures. "This way, the power user and the occasional browser will typically count equally under our system."

The debate is more than academic. Browser makers - except perhaps Microsoft , whose Internet Explorer has been in a long, steady decline - regularly cite data from NetApplications and other metrics companies to prove that their software has snatched share from rivals.

Mozilla has been most aggressive in promoting its share gains - most recently last November when Firefox turned five - as it surged from just 10% of the usage pie in 2006 to nearly 25% in late 2009.

Since then, however, Mozilla has been relatively quiet as its NetApplications-measured share stalled, dipped and then climbed slightly. Last month, Firefox accounted for 24.6% of all browsers by NetApplications' estimate.