Mozilla is looking at dropping support for Windows 2000 and versions of Windows XP without at least SP3 installed in early 2010.
The idea is hardly a surprise, as the extended support period offered by Microsoft for the operating systems is ending on 13th July 2010, and would see the requirements for Gecko 1.9.2 raised – Firefox’s rendering engine.
Firefox 3.5 (aka 3.1) are built using Gecko 1.9.1, so the change will not affect users on current or next version of Firefox.
Full discussion on the issue can be read on the mozilla.dev.planning newsgroup.