First, go to Start->Programs->Accessories->Notepad. Type the signature text you want to appear at the end of each message. Choose File->Save. From that window, choose your Desktop (or other location which you can remember), and name the file "signature". Exit Notepad.

Start up Thunderbird and choose Tools->Account Settings.

On the right, check the box Attach this signature and then click the Choose button.

Choose your signature.txt file that you created (by navigating to Desktop or the other directory you saved it in). Click Open. Click OK.