Mozilla believes more PC users run Firefox than Google's Chrome web browser, despite web metrics firms reporting otherwise.

According to measurement vendors such as Net Applications, Chrome's browser usage share gains now regularly outpace those of Firefox.

In April, Net Applications' 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 Net Applications' reckoning; Firefox added 0.8 of a percentage point in the same period, again one-eighth as much.

However, Asa Dotzler, Mozilla's director of community development, sees it differently.

Rather than measure usage share - which in Net Applications' 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.

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 NetA pplications 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 Net Applications 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 percent of the usage pie in 2006 to nearly 25 percent in late 2009.

Since then, however, Mozilla has been relatively quiet as its Net Applications-measured share stalled, dipped and then climbed slightly.

Last month, Firefox accounted for 24.6 percent of all browsers by Net Applications' estimate.

See also: Mozilla: Firefox 4 will be 'super-duper fast'