The Blank Canvas Gmail Signature Firefox add-on will automatically inserts HTML signatures into your Gmail messages based on which address you are sending from. Supports up to four signatures for each address you have set up. Works for Compose Message and Reply/Forward.

The Blank Canvas Gmail Signature Firefox add-on, once installed, you get a new drop-down menu that lets you select one of your four custom signatures. These can be managed directly within Gmail, and come with an editor that shows you a live preview of whatever HTML you drop in. There are four preset signatures available for you with nicknames like personal, business, and family, all of which can be renamed to suit the type of signature you’ve set up.

Blank Canvas Gmail Signatures  [via]