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

Question

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.

Solution

I have no experience using them in thunderbird, but S/MIME certificates are issued by certificate authorities. This often involves some sort of payment and validation (of sorts) of your identity / existence. For practical purposes, at least startSSL seems to be issuing them for free. Many organizations (companies, schools, etc) may be able to issue such certificates for their employees/other associates.

Comments

[Jacques]

As an updated for this comment that was posted back in 2017, as of January 1st, 2018, StartSSL "does not issue any new certificate from StartCom name roots"[1].