Computer Configuration > Administrative Templates > Firefox > Certificates
Both
HKLM\Software\Policies\Mozilla\Certificates
If this policy is enabled, Firefox will install the listed certificates into Firefox. It will look in %USERPROFILE%\AppData\Local\Mozilla\Certificates and %USERPROFILE%\AppData\Roaming\Mozilla\Certificates. If this policy is disabled or not configured, Firefox will not install additional certificates.