enigMail
For some reason I decided to google GPG thunderbird tonight and started reading about a plugin called enigMail. After I spent about 10 minutes reading about the plugin and how it worked with GPG I decided to give it a go. I downlaoded and isntalled the latest version of GPG (1.4.2.2) which I had to obtain from a mirror and then downloaded the enigmail plugin for thunderbird. After installing the plugin and restarting thunderbird I went through the wizard that enigmail provides. Everything went seemless and before I knew it I had my own PGP public key and thunderbird all set to send emails with all of my accounts. I must say after trying to do the same thing with outlook express many years ago this process has improved immensely. Google enigMail and GPG for some quick tutorials.