To my mind, arguments about the exact market share of Windows 98, and the comparative release dates of the various OSes we support are irrelevant. The key point is that after July 11th, 2006, Microsoft will no longer be providing even critical security updates for any of the Win 9x family. And, because it’s a proprietary operating system, no-one else can do so either.
I argue that continuing to support these operating systems in a browser or other Internet-facing product after vendor security support ceases is actually irresponsible, because it gives users the idea that they can continue to safely use those operating systems for surfing the Internet.
July 11th 2006 will be, if all goes to plan, around the time of the final release of Firefox 2. So if we have a strong commitment to the security of our users, why shouldn’t we intentionally disable Windows 9x support in that release as well, even if it’s not necessary for technical reasons?