There are some issuing bodies that will give you a free certificate, as listed in this Mozillazine article here. One company that offers them for no charge is Comodo and another is startssl as mentioned by Taneli, so those two are two decent possibilities. Personally, I used the Comodo certificate for a while until I had to move to OpenPGP. The Comodo one is guaranteed to work with s/mime compliant applications and their information page mentions thunderbird.

Thunderbird by default has great support for S/MIME signing and encryption. As noted in another mozillazine article, it is relatively quick to setup, although it is advised to set a master password as well to protect the use of your certificates. Importantly, you need to do this before importing your certificate into thunderbird.

• You need to import your certificate into thunderbird by going to edit > preferences > advanced > certificates (first screenshot).


• You then need to add your certificate to your Thunderbird mail account (see second screenshot) by going to account settings and clicking the security tab. All this is simple to setup and to use once you have your certificate.


• The security options are clearly visible when composing a mail (see third screenshot) and you can add your signature or fully encrypt the mail.

These screenshots are from thunderbird 14.

(Note: I have temporarily removed the enigmail OpenPGP addon from my copy of thunderbird so that the menus in my screenshots should look the same as in your thunderbird.)