To export a S/MIME certificate from firefox follow the instructions below:
- Open the Firefox browser and click the Options Menu button at the top-right corner, then select Settings
- Select Privacy & Security from the menu on the left
- On the Privacy & Security tab, scroll down to the Certificates section, and click View Certificates
- In the Certificate Manager window, select the Your Certificates tab, then select the certificate you wish to back up. Click Backup…
- 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.
- 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.
- Click OK to export your Sectigo Personal Authentication certificate.
Source: Sectigo’s Knowledge Base