WiseStamp is a Firefox addon which lets you add attractive and beautiful email signatures directly into your email compose box. You can select from different themes, add social links and a short description of yourself. You can add links to other sites and even add an image of yours. The font and its color is also left upto you. Currently it doesn’t work with all email services, but with the most popular ones namely Gmail, Yahoo mail, Hotmail and AOL Mail.

You can download it from here -
Code:
http://www.wisestamp.com/download/wisestamp-1.1.xpi
- it's an direct download link.