Do What I Say!
When I set Mozilla Firefox as my default browser, I really mean it. I don't mean that it should work properly for everything but a couple of your applications (specifically MSN messanger, but I've seen it happen in others as well), but in fact, it should follow the definition of default browser. That is, it should be the browser that is chosen by default.
I realize your programmers have probably hard coded some programs inside your apps, as your brainwashed minions could not possibly imagine that anyone would ever want to use anything but your buggy, insecure and spyware ridden browser, but the inclusion of a program that changes the defaults of what apps are used in Windows XP should have been a hint.
Even though I've used the 'set program defaults' program and set as default in Firefox, and it still doesn't work... I'm sure it'll be fixed in Service Pack 2, 3, XP Reloaded, or Longhorn. Or whatever comes after. Honest, I do.