Before you can use Thunderbird's built-in Signature feature you must create a signature file in a text editing program. You can use Microsoft Word or any other program that allows you to save a plain text (.txt) file (TextEdit, WordPad, Notepad, etc.). Then you will use the Account Settings in Thunderbird to select the file you created.

Open Microsoft Word or Notepad

Type the text you would like to appear as your signature.

Select File, Save As...

Select a place you will be able to easily find the file, your M:\ drive is a good place.

Select the Text Files (*.txt, *.ini) option from the "Save as type:" drop-down.

Type a name for the file (such as "Signature") and click [Save].

If you get a warning about losing formatting when saving as plain text, click [Yes] or [OK].

Select Tools, Account Settings...

Click to check the box next to Attach this signature:

Click the [Choose...] button.

Navigate to your signature file (Signature.txt) and click [Open].

Click [OK] in the Account Settings Window.

When you compose a new message (via the Write button, one of the Reply options or Forward) your signature will be automatically inserted in the message with a separator line of 2 dashes (--).

To update your signature file open the Signature.txt file in Word or Notepad, make your changes and Save. You do not need to update Thunderbird since only a link to the external file is stored.