Slideshow: The Web Browser Turns 15: A Look Back

Firefox reached the 25% milestone on Sunday, said Vince Vizzaccaro, executive vice president of California-based Net Applications, which measures browser usage by tracking the machines that visit the 40,000 sites it monitors for clients.

"We always thought that Firefox would be in a great position to compete with Microsoft's Internet Explorer if it made 10%," Vizzaccaro said today. "Now one in four people globally are browsing the Internet with Firefox."

Mozilla passed the 10% market share mark in March 2006, said Vizzaccaro.

The move past 25% wasn't a surprise. Last month, for example, Net Applications estimated Firefox's share as slightly over 24%.

Mozilla also touted the milestone. Mitchell Baker , the former CEO of Mozilla Corp. and the current chairman of the Mozilla Foundation, mentioned the 1-in-4 figure in a post to her personal blog earlier this week when she trumpeted Firefox's fifth-year anniversary.

Mozilla released Firefox 1.0 on Nov. 9, 2004.

Asa Dotzler, Mozilla's community coordinator, echoed Vizzaccaro's take on the significance of Firefox's climb. "Before we launched Firefox 1.0, the people on the team, the eight or 10 of us, said that if we could get to 5%, we would stay alive and stay meaningful. If you don't get to 5%, you don't have a seat at the table."

As Firefox surpassed that share, Mozilla upped its goals, Dotzler continued. "When we got to 10% globally, closer to 20% in Europe, we knew that then we have enough of a platform, not just a strong seat at the table but a strong seat, that when we sat down at standards bodies, we were on an equal footing with IE. We had some authority that our large user base gave us," he said.

As Firefox's share grew, Mozilla's ambitions did, too. "When we reached 20%, we became the favored browser for anyone who understood what a browser was," Dotzler said.

For the week of Nov. 1 through Nov. 7, Firefox accounted for 25.1% of all browsers, said Vizzaccaro. Internet Explorer (IE), meanwhile, led all rivals with 63.3%, while Apple's Safari, Google's Chrome and Opera Software's Opera followed, in that order, with 4.4%, 3.9% and 2.3%, respectively.

But while Firefox may have the second spot locked up for now, it faces increased competition from Chrome . Last month, for instance, Chrome's market share gains outpaced Firefox's increase for the fourth time in the last year. If Chrome sustains its average growth rate of the last three months, it will reach a 5% share in March 2010, six months ahead of Google's announced goal of September 2010 . Google's next objective after that is to make 10% sometime in 2011.