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1. Compose signature in new message (only signature).

2. Save as html.

3. Edit -> account settings -> import signature from file 

4. Removing the signature separator:
If you don't want a "-- " signature separator (the divider between the message body and the signature) use the config editor to change mail.identity.default.suppress_signature_separator from false to true. This requires Thunderbird 5.0 or later. Go to Options → Advanced → General and press the Config Editor button. Enter suppress_signature_separator in the Filter: field and press return. Find mail.identity.default.suppress_signature_separator in the results and double click on false in its Value column to toggle it to true, then close the config editor and press the OK button. [1]