You may ran into the following issue with Firefox browsing a SSL Web site using self generated certificate :
The cause is that the self generated certificate of the site you are browsing have the same serial number of a previously stored certificate (another Web site you already visited and manually added an exception for).
To resolve this matter, you need to delete the Firefox certificate database file. Simply delete the “cert8.db” file under your profile under that path :
C:\Documents and Settings\<username>\Application Data\Mozilla\Firefox\Profiles\<profile_name>