Mozilla has a and a where people can keep track of the total. On Thursday afternoon, the feed showed more than 999,180,000 downloads, with about 15 more happening each second.

Mozilla said initially that it expected to hit the billion mark some time over the weekend. An hour later, as the news trickled out and the pace of downloads increased, Mozilla revised its estimate to Friday. An enthusiast Web site with a "" predicts it will reach a billion on Friday at noon GMT.

The figure includes all versions of Firefox since the first release in 2004. If a single user downloaded multiple copies for different computers, they are each counted in the total. And if a user goes to the Web site to download an update to an existing version, instead of waiting for the automatic download, that is counted as well. Automatic updates are not included in the total.

So the figure does not mean that 1 billion people are using Firefox. Still, it's a significant achievement for a piece of software that was unknown to most of the world just a few years ago, and one that has had to compete with Microsoft's Internet Explorer, which ships free with every Windows PC.

Figures from earlier this month showed Firefox having just under a third of the global browser market, at 31 percent. Internet Explorer led the field with 60 percent, while Safari, Chrome and Opera each had less than 5 percent, according to .