The Firefox Web browser is fast approaching its billionth download and is likely to hit that milestone some time on Friday.

Mozilla has a Web site and a Twitter feed 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 “Firefox Download Guesstimator” 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%. Internet Explorer led the field with 60%, while Safari, Chrome and Opera each had less than 5%, according to Statcounter.

Firefox is stronger in Europe, where it has 40% of the market to IE’s 47%. In Asia, Firefox has 23% to IE’s 72%. In Antarctica, Statcounter says, the browsers are neck and neck.

Mozilla plans to launch a Web site Monday, at www.onebillionplusyou.com, where it will provide more information on the achievement.

Microsoft is taking the unusual step of rushing out two emergency security patches ahead of its regularly scheduled updates on August 11.

The patches will include a critical fix for Internet Explorer as well as a related Visual Studio patch rated “moderate” urgency by Microsoft.

“The Internet Explorer bulletin will provide defense-in-depth changes to Internet Explorer to help provide additional protections for the issues addressed by the Visual Studio bulletin,” Microsoft said in a blog posting late Friday.

The patches are set to be released on Tuesday at 10 am West coast time.

Microsoft didn’t say exactly what it was fixing. The company typically doesn’t rush out these “out-of-band” emergency updates unless the bug is being exploited by cybercriminals; however, in this case the flaws being patched are not being leveraged in attacks, according to Microsoft.

The problem appears to lie in a widely used Windows component called the Active Template Library (ATL). According to security researcher Halvar Flake, this flaw is also to blame for an ActiveX bug that Microsoft identified earlier this month. Microsoft issued a kill-bit patch for the problem on July 14, but after looking into the bug, Flake determined that the patch didn’t fix the underlying vulnerability, so new attacks are possible.

Whatever the issue, the new patch should be a top priority for IT staff next week. “When Microsoft goes to an out-of-band patch, I think it’s sensible for people to apply it,” said Roger Thompson, chief research officer with AVG Technologies.

Microsoft didn’t give a reason for the rushed update but it may be trying to stay ahead of any public disclosure at next week’s Black Hat security conference in Las Vegas. The emergency updates are set to be released the day before the Black Hat Briefings, where researchers Mark Dowd, Ryan Smith, and David Dewey will talk aboutbrowser security issues.

According to security experts, thousands of Web sites have been used to launch on-line attacks that exploit the ActiveX vulnerability patched in July. The flaw was first reported to Microsoft more than a year ago.