This isn't a virus. It's an application. Security Essentials warned me that Firefox wanted to make an exception on the firewall, and I allowed it. It was my fault. I have never seen a browser application ask to breach the firewall. I should have known better. It will never happen again.
I could be mistaken, but on my machines, Microsoft Security Essentials provides a firewall and some mal-ware detection, but it is not a replacement for an antivirus program.
McAfee, Symantec, and AVG all run fine with Micrsoft Security Essentials.
I’ve also recently updated a couple machines to FireFox 4.01.
Neither needed nor requested any firewall updates. Microsoft Security Essentials didn’t make a peep about it on either system. It seems to me that you were very likely installing an infected or corrupted copy, or that another virus was already on your system, just waiting for the chance to change your firewall settings...
“You can’t run two antivirus programs. They each see the other as a virus.”
I don’t think so. I’ve been running Avast Free and Malwarebytes (paid) for a long time... on multiple machines.