Mozilla Firefox uses its own SSL certificate store contained in an NSS database instead of the OS certificate store.
The NSS tool certutil can be used to read and to write to an NSS database.
Note that the NSS database directory may contain a
cert_override.txt file. In order to start clean,
it may be necessary to terminate all related processes (e.g., Firefox)
and to edit or delete the cert_override.txt
file.
Resources: