This Firefox add-on allows you to easily maintain up to four separate rich HTML signatures for each email address you have set up in your Gmail account.

The add-on's home page is http://blankcanvasweb.com/pages/detail/id_9/n_gmail_signatures/

Features:

• Works for Compose Mail, Reply, and Forward
• Support for single or multiple email addresses
• Create a different signature for each email address you use in your Gmail account
• Supports up to four signatures per address
  
• Real-time signature preview while editing signature HTML
• Signature automatically inserted into message above (or optionally below) quoted text
• Support for special characters
  
• Easy setup and configuration interface