The Firefox Team announced that they won’t support Windows 98 and Windows ME by the time they release Firefox 3.0. The reason would be that Microsoft doesn’t support 98/ME anymore.
Firefox 2.0 will run on Win98 / Me. However, people still using the mentioned OS should consider upgrading their PC to run Windows XP. Security Holes won’t be fixed anymore, neither in Firefox nor in Internet Explorer 6.0.
It’s assumable that Opera will follow this trend, too.