Navigate to the certificate manager window. Depending on your version of Mozilla or Firefox, the path to this window could be any of the following:
Edit->Preferences->Privacy & Security->Certificates->Manage Certificates
The certificate manager window appears, open to the Your Certificates tab.
In Mozilla or more recent versions of Firefox, click the Manage Certificates button. In some earlier versions of Firefox, click View Certificates.
Click the Import button.
In the File Name to Restore file chooser dialog, select the .p12 certificate file, then click Open.
In the Prompt dialog, type an account password for the Master Password for the Software Security Device.
This password is specific to your browser account and is assigned by the browser profile owner. A common convention is to use the same password as the login account on the client machine.
In the Password Entry dialog, type the certificate password.
This password is used to protect the client certificate. If you are using a registry-generated certificate, type the password that you specified on the User Authentication Details page.
An Alert dialog with the message: “Successfully restored your security certificate(s) and private key(s)” appears.
Close the Certificate Manager and Preferences/Options dialogs.
After you import the certificate, you are ready to log in to the registry. See To Log In to the Registry for details.