Problem has only started since I upgraded to BitDefender 2013 from BD 2012. When I've used the "add exception" I've gotten a very basic page. Example- tried to log in on my att-Yahoo email and got the TCU message. Did the "add exception" and now instead of the sign in screen I've always had it is now a very basic left justified page. Why? When I sign out of my att-Yahoo home page I get the TCU for cprox.att.net. This even happened when I tried to sign President Obama's B-day card! (A Republican plot?) I think there is a problem between Firefox and BitDefender 2013.
Set BD 2013 to stop managing the SSL certificates in Firefox. These security suites are really gaz factories nowadays. You can try to delete cert8.db in your profile (make a bac-up copy before deletion).