One in every four people on the Internet are now using Mozilla's open source Firefox browser, a web metrics company said.

Firefox reached the 25% milestone on Sunday, said Vince Vizzaccaro, executive vice president of 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.

Mozilla also touted the milestone. Mitchell Baker , the former CEO of Mozilla and the current chairman of the Mozilla Foundation, mentioned the one in four 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 9 November, 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.

According to notes posted Wednesday on the Mozilla site, about a quarter million people are running Firefox 3.6's preview. Mozilla has tentatively set the launch of 3.6's release candidate for 26 November, and is shooting for a final build before the end of the year.

Firefox can be downloaded from Mozilla's sites in versions for Windows, Mac OS X and Linux.