Mozilla has announced today that Firefox 12, slated to be released June 5th, will be the last supported version for Windows 2K, and XP RTM/SP1 systems. The development team has been given the orders to begin upgrading to Visual Studio 2010, even thought less than half a percent of Firefox users remain on these outdated systems.
Source: http://weblogs.mozillazine.org/asa/archives/2012/01/end_of_firefox_win2k.html
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For a number of years we've held off on updating our Windows toolchain to a newer version of Visual Studio, and in so doing preserved support for Windows 2000 and Windows XP RTM and SP1. Firefox developers and the 99.6% of our Windows users have paid a price for this support, though. Our developers have not been able to take advantage of new compiler features 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. Our users have have suffered a slower Firefox than would be possible as both direct and indirect results of moving to a more modern compiler.
Source: http://weblogs.mozillazine.org/asa/archives/2012/01/end_of_firefox_win2k.html