If you start a message in HTML mode, choosing Options->Format->Plain Text Only switches to plain text mode but the compose window does not switch to a monospace font. Starting a message in plain text mode uses the correct font.
If you start a message in HTML mode, choosing Options- >Format- >Plain Text Only
switches to plain text mode but the compose window does not switch to a
monospace font. Starting a message in plain text mode uses the correct font.