Mozilla's Asa Dotzler has announced that Firefox will be dropping support for Windows 2000 and pre-SP2 Windows XP, starting with Firefox 13. New releases will only work on Windows XP SP2+, as well as on Vista and Windows 7.
Mozilla has had to drop support for these older operating systems in order to be able to switch to Microsoft Visual Studio 2010, which doesn't offer support for them either.
Switching from VS 2005 will enable the Firefox team to perform some optimizations
that will benefit the browser in the long run, either via an instant performance boost or via a productivity boost from Mozilla software engineers and contributors.
For those stuck with Windows 2000, Mozilla recommends a browser like Opera, as the only viable alternative. Those running Windows XP can update to the latest service pack, of course.