Mozilla's Firefox browser is on track to hit the 20 percent market share mark next month, a web metrics company has said.

Firefox boosted its share by 0.6 percent in May, accounting for 18.4 percent of the browsers used during the month and putting it within shouting distance of a major milestone, according to Net Applications.

"Firefox is trending to hit 20 percent market share some time in July," said Vince Vizzaccaro, the company's executive vice president of marketing, in an email.

May's share was a Net Applications record for the open-source browser, and resumed gains that had been interrupted in April when its share dropped slightly. The one-month increase was also the largest by Firefox since March 2007.

"Firefox is surging again, but their gains when reviewed over time aren't out of the ordinary," said Vizzaccaro. "Other than the stagnation Firefox experienced in mid-2007, their growth has been fairly consistent."

Vizzaccaro was referring to several months last summer when Mozilla's flagship browser slipped in market share, including May 2007.

Although Vizzaccaro didn't pin all of Firefox's increase on a change last month to its update dialogue, he did note the new approach. "Mozilla has implemented a change in Firefox 3.0 RC1 [Release Candidate 1] where the installation now has a checkbox that defaults to making Firefox your default browser," he noted.

Vizzaccaro refused to ding Mozilla for the practice. "The option is clearly displayed and labelled, unlike Safari, which misleadingly labelled the Safari install as an 'update' [but] since correctly changed to an 'install.' However, this practice is a break from the traditional practice browsers employed of defaulting this option to off."

Gains, as in the past, came at the expense of Microsoft's Internet Explorer (IE), which dropped more than a percentage point in market share during May, to 73.8 percent. Two years ago, IE accounted for 84.1 percent of all browsers used to access sites operated by Net Applications' customers.

"Apple is continuing to see Mac market share growth [and] with IE not being available on the Mac any longer, we're seeing Safari and Firefox gaining market share by riding the gains of the Mac," Vizzaccaro said, explaining the growth for Safari and to a lesser extent, Firefox.

Vizzaccaro also answered questions about the reliability of May's data; early last month, Net Applications discovered that a large-scale online marketing campaign aimed only at IE users had skewed April's numbers, and erroneously claimed that both Firefox and Safari dropped significantly in market share. It recalculated the data and issued an update May 7.

"Each report now has a QA stamp on the bottom once it's been reviewed and confirmed," said Vizzaccaro. "We want to continue to get our market share data out as quickly as possible, and this gives our readers a way of knowing when the data has been confirmed."