Its MS trying to be crafty again, apprently they broke the new version of .netframework so it won't work properly on anything non MS i.e trying to force people to use explorer.
Its a know issue with FF4 and dunno why they don't fix it. I did the same thing and upgraded to ff4 and the .net install went crap. I just rebooted and FF4 works fine.
This is being discussed on the interwebz in many places, just google it.
Read the comments below to get an idea
https://addons.mozilla.org/en-us/firefo ... t/reviews/Hope that helps