If you have not already done so, we recommend setting up a master password for Firefox, which will also protect the private key portion of the certificate you are importing.

To do this, go to Settings => Privacy & Security => Passwords and enable Use master password.

Please select a master password that you will not forget. Otherwise, you will not be able to access the data protected by it.

In the Settings menu, scroll down to Privacy & Security, then scroll down to Certificates and select Show certificates.

In the Certificate Management window, locate your certificates and click Import.
In the window that opens, select the newly received *.p12 file and enter the password used to protect the *.p12 file.
For newly issued certificates, this should be the password you chose when applying for/creating your user certificate (*.p12 file).

The newly imported certificate is now displayed under Settings => Privacy & Security => Certificates => Show certificates, where it can be deleted later if no longer required.