I have always used a signature in my accounts in Thunderbird. Now that I'm using SeaMonkey I also use a signature. There has always been a "signature separator" in emails I composed and sent out. I finally decided to find a way to remove it. The separator looks like this "-- " without the quotes. I finally found this on mozillaZine.
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. If you are using Windows go to Tools → 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.
For those who use SeaMonkey, you can accomplish the same thing using config editor in the browser, then searching for mail.identity.default.suppress_signature_separator and changing the value to true.
