From MozillaZine Knowledge Base

A signature is a block of text that is added automatically when you compose a new mail message or reply.

You can include a signature for each of your Mozillla Suite/SeaMonkey Mail account identities. Each signature can be plain text or HTML and is stored in a separate file on your computer. Any signature file can be used by more than one mail account identity.

1. Create a text or HTML file with your signature and store it on your hard drive.
2. In Mail, go to "Edit -> Preferences -> Mail & Newsgroup Account Settings".
3. Select a mail account identity.
4. Check "Attach this signature".
5. Click "Choose..." to locate the signature file you previously created.

See also

• Signatures - Thunderbird