Put a signature in the Settings 'Email Signature' box at the bottom of the screen.
Compose a message and hit 'Add Signature' and I don't get what I put in the settings box for 'Email Signature'. I get the text that appears in the block right at the very base of the Settings screen titled:
"Identities - Set up From addresses and Customize signatures"
....which appears to be a nonsense section at the end of Settings that won't click or edit in any way....