To create a Thunderbird signature on a computer running Windows:

1. Open a text editor application, such as WordPad.

(You should be able to find Wordpad by going to the Start menu, selecting All Programs, selecting Accessories, and selecting WordPad.)

2. In Wordpad, type what you want to have appear in your signature. Common items to include are your name, your e-mail address, and other contact information you wish to share with any person you send e-mail to.
3. Go to the File menu and select Save As.... In the window that appears, in the File name: field, name your file (a suggestion would be "signature010107" if you created the signature on January 1, 2007).
4. Note where you are saving the file to. In most cases, saving the file to your My Documents folder is best.
5. In the Save as type: field, select Text Document from the list. Skipping this step will make your file unreadable by Thunderbird.
6. Click the Save button.
7. If you are given a warning that saving the file in a Text-Only format will remove all formatting, click Yes. Thunderbird requires that your signature file be a text document.
8. Close WordPad or the text editor application you were using.
9. Open Thunderbird.
10. Go to the Tools menu and select Account Settings.
11. Click once on your e-mail address in the left column.
12. Check the box next to Attach this signature.
13. Click the Choose... button
and browse to the text file that you created.
14. Click Open to select the file.
15. Click OK. Your new signature file should now be included at the bottom of every e-mail message you compose.
16. Send a mail to yourself to see the result. If you don't like the way the signature appears, you can go back and edit the text file.

To create a Thunderbird signature on a computer running Mac OS X:

1. Open a text editor application, such as TextEdit.

(You should be able to find TextEdit by double-clicking on Macintosh HD, then double-clicking on Applications, then opening TextEdit.)

2. In TextEdit, type what you want to have appear in your signature. Common items to include are your name, your e-mail address, and other contact information you wish to share with any person you send e-mail to.
3. If you are using TextEdit, go to the Format menu and select Make Plain Text. This will format your signature correctly for Thunderbird.
4. Go to the File menu and select Save As.... In the window that appears, in the Save As: field, name your file (a suggestion would be "signature010107" if you created the signature on January 1, 2007).
5. Note where you are saving the file to. In most cases, saving the file to your Documents folder is best.
6. If you are using TextEdit, make sure that the box next to If no extension is provided, use ".txt" is checked. In the pop-up next to Plain Text Encoding: Western (Mac OS Roman) should be selected.Skipping this step will make your file unreadable by Thunderbird.
   (In most other text document applications other than TextEditor, in the File Format: field, select Text Document from the list.)
7. Click the Save button.
8. Quit TextEdit or the text editor application you were using.
9. Open Thunderbird.
10. Go to the Tools menu and select Account Settings.
11. Click once on your e-mail address in the left column.
12. Check the box next to Attach this signature.
13. Click the Choose... button
and browse to the text file that you created.
14. Click Open to select the file.
15. Click OK. Your new signature file should now be included at the bottom of every e-mail message you compose.
16. Send a mail to yourself to see the result. If you don't like the way the signature appears, you can go back and edit the text file.