Hi,

Click Options then Advanced. There is a button "view Certificates" click on it and then choose the server tab. This will list the servers that have exception, you can then click on the ones you want to remove and delete them.

This is from Firefox 3.5.1 and windows XP, but it should be the same or close enough to get you going in the right direction.

See if you can find that exception in the Certificate Manager: Tools > Options > Advanced > Encryption: Certificates: View Certificates

Rename the file cert_override.txt (cert_override.txt.old) in the Firefox Profile Folder This will remove all exceptions that you have stored in the Certificate Manager.