Unable to send or receive emails.

For regular users: Your email client like Gmai, Outlook,thunderbird etc.. and your provider handle all the necessary security certificates in the background to keep your connection safe.

You only need a personal S/MIME certificate if you want to read heavily encrypted, top-secret emails sent specifically to you. For custom email servers: If you run your own private mail server, you must install an SSL/TLS certificate so other servers can securely connect and deliver mail to you without blocking it.

Hope this helps.

Please post a screenshot of the error. https://support.mozilla.org/kb/how-do-i-create-screenshot-my-problem