get rid of dash line in thunderbird signature

 

1 Open Mozilla Thunderbird. 

2 Open the "Tools" menu. 

3 Select "Options," then select "Advanced." 

4 Select "Config Editor." A popup will appear warning you that making changes may void your  warranty or affect the "security, stability and performance of this application." Click the "I'll be careful, I promise!" button to continue. 

5 Find the "mail.identity.default.suppress_signature_separator" entry and double-click it. This will change the entry from "false" to "true" and change the entry to boldface type.