There are parties, e.g. enterprises, that uses S/MIME certificates for e-mail encryption. To my understanding Thunderbird has support by default for S/MIME. But I have not found a way to obtain a personal S/MIME certificate to be used on a Ubuntu system. Thus my question:

How to obtain a S/MIME certificate for e-mail encryption?

Moreover, is it correct to believe that Thunderbird has support by default for S/MIME?

I am fully aware of PGP, GnuPG and OpenPGP for public-key cryptography and secure e-mail communication. In my opinion, it is very useful that OpenPGP is installed by default on Ubuntu systems. But I need to find a method to communicate securely with parties that use S/MIME and not PGP/GnuPG/OpenPGP.