It is possible to automatically add rich HTML signatures to your Gmail messages if you use the Blank Canvas Gmail Signatures add-on for Firefox which automatically inserts HTML signatures into your Gmail messages based on which address you are sending from. Works for Compose Message and Reply/Forward.

The add-on's home page is at http://blankcanvasweb.com/pages/detail/id_9/n_gmail_signatures
I try to put out updates frequently and do take user feed-back into account as much as my schedule allows. You can get a lot of information about the add-on itself,  setup, and HTML signatures in general on that site. There are also forums if you would like to submit bug reports, comments/suggestions, or ask questions.

Jerome Dane

Please note that this is not an official Gmail product.