Sounds like may be you are composing messages in plain text rather than HTML, switch to HTML and that should solve it. You can also change the default from your settings. Preferences > Composing Messages. Under Main Options: Compose HTML messages
An option to remove the standard sig separator (the 3 dashes) was added in version 1.2 so assuming you are using that then you can change it in Preferences > Composing Messages. Under Signature Options: Force standard separator in signatures