To export a S/MIME certificate from firefox follow the instructions below:

  1. Open the Firefox browser and click the Options Menu button at the top-right corner, then select Settingssmime export
  2. Select Privacy & Security from the menu on the left
  3. On the Privacy & Security tab, scroll down to the Certificates section, and click View Certificates
  4. In the Certificate Manager window, select the Your Certificates tab, then select the certificate you wish to back up. Click Backupcertificate manager
  5. Your certificate will be exported to a PKCS12 file. To learn more about certificate formats, check our comprehensive SSL formats guide. Please create a name for this file and specify where you want to save it.save certificate
  6. Next, you must create a password to protect your PKCS12 file. Remember this password because you need it if you import the certificate into another browser or mail client.create password
  7. Click OK to export your Sectigo Personal Authentication certificate.success alert

Source: Sectigo’s Knowledge Base