If you have not already done so, we recommend setting up a master password for Thunderbird (formerly known as the master password), which will also protect the private key portion of the certificate you are importing. To do this, go to Settings (gear icon) => Privacy & Security => Passwords.

In the Privacy & Security menu, scroll down to Certificates => Manage certificates.

In the certificate management, you will see your certificates and select Import.

In the window that opens, select the newly received *.p12 file.

Then enter the master password for Thunderbird.

The password used to protect the *.p12 file must be entered as the password for the certificate backup.

For newly issued certificates, this should be the password you chose when applying for/creating your user certificate (*.p12 file).

If you have changed your password, you will need to enter the new password.

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