Are FF and later versions of IE really slower than IE6? I remember IE6 with QQ toolbar being literally unusable for me before I gave my then colleague's laptop a lease of life by upgrading her browser to IE7 (sans toolbar) for the ActiveX-dependent extensions and Firefox 3 for everything else.

Mostly I'd assume it's a function of IE-specific code in webapps and pre-XP OS installations that means that IE6 is not only required but necessary for some purposes.