Getting an SMIME certificate
From MozillaZine Knowledge Base
You can get an S/MIME certificate that is trusted by the built in authorities in Firefox and Thunderbird from Thawte via their personal email certificate program for free.
Initially, the certificate will not have your name, but only your email address. After you have received 50 trust points, you can have your name on your certificate. You earn trust points for veryfying your identity to members of the Thawte Web of Trust.
For more details and to sign up and get your free personal S/MIME email certificates, click here.