Here is an extension for Firefox from the same guy that wrote the Gmail FileSystem that allows you to send and receive encrypted e-mail just like Mail and Thunderbird.

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.