There should be permanent block entries visible for the DigiNotar certificates in the Servers tab in the Certificate Manager *Firefox > Preferences > Advanced : Encryption: Certificates - View Certificates There are more certificates that are blocked besides the certificates from DigiNotar. If those are missing then try to delete the cert8.db file in the Firefox Profile Folder and you can check the content of a possible cert_override.txt file. *http://kb.mozillazine.org/Profile_folder_-_Firefox You can use this button to go to the Firefox profile folder: *Help > Troubleshooting Information > Profile Directory: Show Folder