How to Add Signatures to Emails
To automatically add a signature to emails that you send from Thunderbird (Mail Client), you must create a signature template in Settings.
Open Thunderbird and select your email address from the tree on the left-hand side of the screen. Under the Accounts heading click View settings for this account.
In the Account Settings window select your email address in the tree on the left-hand side. Enter your signature in the box labeled Signature text. You may format your signature with HTML by checking the box next to Use HTML. Press the OK button when done.