Playing with FreeIPA? Recreating the CA? Firefox complaining about SEC_ERROR_REUSED_ISSUER_AND_SERIAL? Can’t find the certificate via Preferences / Privacy & Security / Certificates?

Try deleting it via CLI:

certutil -L -d <folder/to/cert8.db> | grep -i ipa
certutil -D -d <folder/to/cert8.db> -n <nickname>