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Slashdot is reporting that the patch to remove support for Pre-Windows 2000 platforms has been checked in on the trunk. As per the Trunk and Branch Management Plan, Firefox 3 will be based off Gecko 1.9.
Gervase Markham has called for dropping Windows 9X/ME support from Firefox 2 as well, noting that even critical security updates will not be released for them after July 2006.
http://www.mozillazine.org/talkback.html?article=8887