I made a post in 2009 explaining how to digitally sign an e-mail with FireGPG. Unfortunately, “FireGPG is discontinued” and no longer supports Gmail. So, we need to do it differently now.
Objective: To sign your out-going emails through an on-line email provider such as Gmail or Yahoo mail.
Operation Systems: Any.
Browsers: Firefox or Chrome.
Add-on or extension for the browser: Mailvelop (get it from http://www.mailvelope.com/)
It works with: Gmail, Yahoo mail, Hotmail, and most other on-line, email providers.