You want to send and receive encrypted email conveniently with Mozilla's Mail & Newsgroups application.
Use Enigmail from enigmail.mozdev.org for GnuPG encryption support. S/MIME is also supported natively within Mozilla.
Once you have downloaded and installed Enigmail, compose a message normally, addressing it to someone whose public key is in your GnuPG keyring. Instead of clicking the Send button, notice that your message window has a new menu, Enigmail. From this menu, you choose to encrypt or sign your message, or both, and it is immediately sent.
To decrypt a message you receive, simply view it and Mozilla will prompt for your GnuPG passphrase.
Your Mail & Newsgroups window also has a new Enigmail menu. Explore both menus where you'll find numerous useful options and utilities: generating new GnuPG keys, setting default behavior, viewing the actual gpg commands invoked, and more.
The Enigmail home page is http://enigmail.mozdev.org, and Mozilla's is http://www.mozilla.org.