UPDATE 2018-05-17: web links are updates.
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 online email provider such as Gmail or Yahoo mail.
Written by Eric Bright
Last update on August 29, 2013
2nd updated on August 25, 2011
First published on Feb 3, 2009
[UPDATE: This solution would not work with Gmail any longer, because the Firefox add-on that I was using at the time is not supported any more. Read the instructions for the updated method of signing your email here.]