Sounds like you got spoofed by something pretending to be FireFox. You should get and run an anti-virus program. Microsoft Security Essentials is not nearly enough.
Often, your ISP will provide one for you to download, like McAfee or Symantec. If not, a halfway decent free one can be had here:
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.