How to import a valid copy of your electronic certificate
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To import a certificate in Firefox it is necessary that the copy has been made with a password, since to install it in this browser it is mandatory to enter the "backup password".
Go to "Edit", "Preferences".
Under "Privacy and security", "Security", "Certificates", click "View certificates".
In the "Your certificates" tab, click "Import..."
The extension of a valid copy of an electronic certificate is .pfx or .p12. Select it from the location where it is saved and press the "Open" button.
The browser may ask to enter the master password (it is the password for the certificate store of the Mozilla Firefox browser and by default it is not set). If there are none established, it can be left blank. If no message about the master password appears, continue ahead.
A new window will ask for the password that was set to encrypt the backup. Once indicated, press "Accept".
If everything is correct, the message "Your security certificate(s) and private key(s) have been successfully restored."
The certificate will appear in the "Your certificates" tab. Check the certificate name and expiration date.