This guide will show you how to add or change an email signature in Thunderbird
Once you are ready to set up your email signature, open Thunderbird, at the top right, click the Settings icon and select Options then Account Settings.
On the right side paste your new or updated email signature into the Signature text field. Be sure to select “Use HTML” if it isn’t already ticked.
Click the OK button and click to compose a new email message. Enjoy your new signature!