Firefox comes with a certificate store for identifying certificate authorities (CA). Using these certificates enables the browser to automatically verify certificates issued by Web sites. If a Web site issues a certificate that has not been signed by one of the CAs from the certificate store, it is not trusted. This ensures that no spoofed certificates are accepted.
Large organizations usually use their own certificate authorities in-house and distribute the respective certificates via the system-wide certificate store located at /etc/pki/nssdb. To configure Firefox (and other Mozilla tools, such as Thunderbird) to use this system-wide CA store in addition to its own, export the NSS_USE_SHARED_DB variable. For example, you can add the following line to ~/.bashrc:
export NSS_USE_SHARED_DB=1
Alternatively or additionally you can manually import certificates. To do so, in the menu bar, open the Preferences dialog by clicking Edit › Preferences. Select Advanced › Certificates › View Certificates › Your Certificates › Import and select the certificate to import. Only import certificates you absolutely trust!