Looks like it can be done. All I'm doing is dumping keywords into google, http://www.google.com/search?q=encry...il&btnG=Search

http://richard.jones.name/google-hac...ail-smime.html
Gmail S/MIME provides Firefox users with S/MIME support for Gmail. S/MIME support is integrated directly into Google's Gmail web interface. Reading and sending of encrypted mail is supported. Verification of signatures is not currently supported. There are some interoperability and performance issues which I plan to iron out in later releases.