Let's get this straight: "Raise the minimum requirements to require Windows XP Service Pack 3 or higher," with no benefit, and no rationale other than for breaking compatibility for its own sake? If that's the case, I venture to say that Mozilla has seriously lost its way.
So, Microsoft ditched support for Windows 2000 and Windows XP pre-SP2? So what; the APIs are just the same now as they always have been. If anything, Mozilla should focus more attention to catering to users of OS versions that Microsoft left behind, where they have less competition...and chances are, the users of Windows 2000 are still using the OS that they are because they're frustrated with Microsoft's "support" policies and the further regressions (performance and usability issues, product activation) posed by newer versions of its products.
I'm seriously still bitter about them breaking compatibility with Windows 95 and NT4 a few versions back: One consequence was that the current version of Firefox was no longer capable of running off a version of Windows not unremovably inundated with Internet Explorer and its ilk. Short of a miracle of penetration from the Linux camp, how are we going to wean people off of a steady consumption of upgraded Microsoft products when we get attitudes and potential decisions like this?