Many people don't realize that you can get a free certificate for digitally signing and encrypting email. Thawte is one company that offers a free email certificate service. Once you have the cert its fairly simple to sign or encrypt email.

I'm using Thunderbird as my email client currently, and for some reason it looks like Thunderbird and FireFox don't share a keystore (I think they should, and they may on your computer). I used IE (you could also do this from firefox, if you used it to generate the cert request) and exported the key as a pfx file (go to Tools/Internet Options/Content/Certificates and export the Thawte Freemail cert). Then you can import the key into Thunderbirds keystore (Tools/Account Settings/Security/Manage Certificates). Once your cert is in TBirds keystore, select the key to be used for Signing and Encryption in the Security panel. To sign or encrypt an email select the Security button drop down in the Compose window. In order to send encrypted messages, you need the public key of the recipient.