Mozilla on Friday announced that next month's Firefox 12 will be the last version to run on early editions of Windows XP and the 12-year-old Windows 2000.

The company also reiterated that it will stop serving security updates for 2010's Firefox 3.6 in April.

Starting with Firefox 13, the browser's minimum requirements will be XP Service Pack 2 (SP2). Firefox 13 will not work on Windows 2000, Windows XP RTM (release to manufacturing, the original mid-2001 build) or XP SP1.

Firefox 12, set to ship April 24 and due to be replaced by the next edition on June 4, will be the last that supports the three older Windows.

"This support change allows us to significantly improve Firefox performance on Windows by using a more modern build system," Mozilla said in a Friday post to a company blog.

The decision wasn't a surprise: Mozilla has been discussing the change for at least three years. And the company actually pulled the trigger two months ago, when Asa Dotzler, director of Firefox, explained the firm's reasoning.

"Our developers have not been able to take advantage of new compiler features [in Visual Studio 2010] and have had to struggle to keep valuable optimizations from breaking, including having had to back out and ultimately delay some important new features like SPDY," said Dotzler in late January. "Our users have suffered a slower Firefox than would be possible as both direct and indirect results of moving to a more modern compiler."

SPDY, for "speedy," is a Google-crafted protocol that promises faster and more secure page loading. Mozilla added support for SPDY in Firefox 11, the March 13 release.

By switching to Visual Studio 2010, Mozilla will not be able to build Firefox for the older operating systems, said Dotzler. But it's not as if Mozilla has jumped the gun. Microsoft retired all three editions years ago.

Windows 2000 fell off Microsoft's support list in mid-2010, and XP and XP SP1 were dumped in 2004 and 2006, respectively. Microsoft doesn't even support Windows XP SP2.

The only version of Windows XP still backed by Microsoft with security updates -- including patches for Internet Explorer 6 (IE6), the browser that shipped with the OS -- is SP3, which released in 2008 and has two years of support life left.

Mozilla advised Firefox users still running Windows XP RTM or XP SP1 to migrate to a newer operating system -- Windows XP SP3 is a free upgrade -- and urged Windows 2000 customers to do the same.

Dotzler also steered Windows 2000 users toward a rival. "If you're a Windows 2000 user and you simply cannot upgrade your PC to a more modern Windows version, I'm sure the good folks over at Opera [Software] will be happy to help you out," said Dotzler on Friday.

Opera runs on Windows 2000, but its Norwegian maker recommends XP or later.

Unlike Opera, Google's Chrome is not an option, since Chrome's lower limit is the same as Firefox's, Windows XP SP2.

On Friday, Mozilla also repeated what it had announced the week before, that Firefox 3.6 is nearing support retirement.

Firefox 3.6.28, which Mozilla shipped March 13, is the last planned update for the two-year-old browser. Between now and April 24, Mozilla will only release fixes to 3.6 if developers uncover critical security or stability issues.

Mozilla advised Firefox 3.6 users to upgrade to the current edition, or failing that, to Firefox ESR (Extended Support Release), the build that targets enterprises leery of upgrading browsers every six weeks.

Because Firefox ESR receives only security updates during its 54-week lifespan -- the first edition is based on Firefox 10, and won't appreciably change until November 2012 -- Windows 2000, XP RTM and XP SP1 users can continue to run it through early February 2013.

Gregg Keizer covers Microsoft, security issues, Apple, Web browsers and general technology breaking news for Computerworld. Follow Gregg on Twitter at @gkeizer, on Google+ or subscribe to Gregg's RSS feed . His email address is gkeizer@computerworld.com.

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