From MozillaZine Knowledge Base

Why don't current versions work on Microsoft Windows98 or Microsoft WindowsME?

Firefox3, Thunderbird3, Sunbird0.3 share Gecko 1.9 for display layout. Gecko 1.9 uses the cairo open-source cross-platform graphics rendering library. With cairo, rendering text doesn't work on Windows98, WindowsME, windowsNT.

(Reprieve: While Sunbird 0.3 used Gecko 1.9, Sunbird 0.5 uses Gecko 1.8 for compatibility with Lighting 0.5 on Thunderbird 2.0, so Sunbird 0.5 can be used on windows 98. Be aware of Windows timezone issues in release notes. When Firefox and Thunderbird 3 comes out, expect Sunbird to use Gecko 1.9 again.)

OLD: may be able to build with GDI+ instead of cairo, but requires Microsoft's GDI+ dll. But as of 11 July 2006 Microsoft no longer supports Windows 98 and Windows ME, so Microsoft bugs are not going to be fixed.