One such page, as Ed reports, is the Microsoft KB search page. If you try to hit this page with Firefox (I'm using v1.0PR), you get bounced back to http://support.microsoft.com/default.aspx. For Firefox users, there is a simple solution - just download the User Agent switcher extension, change your user agent string to either Opera or IE, and it works.
Some people might call this a bit of pretty lame programming - I'm possibly not as charitable. But, at the end of the day, given how much better Google is in terms of searching MS content, and how much better a browser FireFox is, I'm just not prepared to give up a far superior browser in order to use a sub-optimal search facility, even if it does allow Microsoft to show the huge numbers of IE-enabled users are hitting their site, and how few folks use FireFox.