The private key for an S/MIME certificate is generated by the browser during certificate enrollment. When the submit button is selected, a key-pair of the selected size is generated. Once generated, the private key is encrypted and stored in the local key database.

S/MIME certificates can be applied only using Internet Explorer on Windows.

note: Safari on Mac no longer supports key generation.

Firefox ESR version 68 and earlier can be used for this purpose.
Firefox has stopped keygen support from version 69 [ released on 3-Sep-2019 ]

https://ftp.mozilla.org/pub/firefox/releases/

Coming Soon!
In an effort to help with Code Singing and S/MIME CSR Generation on browsers that no longer support key generation, Sectigo is currently finalizing a CSR Generation Utility where you will be able to generate the CSR for the following certificate types:

• Email
• Code Signing
• SSL

You will also have the option to:

• Generate a private key on a SmartCard.
• Save the CSR to a file

Should you have any questions or concerns please do not hesitate to contact our Technical Support Team

Related Articles:

• Revocation of Secure Email Certificate
• I get an error message 'A Secure Email Certificate has already been issued for this Email Address', why?