How to get rid of double dash before the signature?
Open the Config Editor: Either Tools -> Options or Edit -> Preferences
Go to Advanced -> General -> Config Editor
Then search for mail.identity.default.suppress_signature_separator
and set it to TRUE by double-clicking it.
Now all new emails are created without the --
before the signature.
I researched this recently and found that they will provide a checkbox in either v3.2 or v3.3 to allow users to elect not to show those dashes. Wait.