I liked Mail.app very much and was very happy with it until I started to have to exchange encrypted emails. Since Thunderbird seemed like the only email client with a reasonably working GnuPG plug-in, I switched about a year ago. In the process, among a few other little annoyances, I started to get a "--" showing up above my signature.

There are amongst the longest threads on the subject with most of it boiling down to an argument between designers - "It is ugly and I want to remove it" and programmers - "Nope". After a few years of complaining, Thunderbird's developers finally gave in and made the "--" optional. You just have to dig very deep to find it.

Thunderbird > Preferences > Advanced > General > Config Editor
set mail.identity.default.suppress_signature_separator to true.