First open Notepad and create your desired signature.
Save this text file in a familiar location on the hard drive. I put mine in Documents. Next open Thunderbird and click on Tools Account Settings.
Keeping the default dialog box open, in Default Identity check Attach this signature: Then browse to the location where you kept the signature text file and click OK.
Now the signature will show up any time you create a new email message.
There are also several cool Add-ons for Thunderbird for further manipulating your email signature.