this works:

Next nightly build starts to support basic features of contenteditable=plaintext-only if you enable dom.element.contenteditable.plaintext-only.enabled from about:config (it needs to be enabled before loading the page) because bug 1921162 will support inserting line break instead of paragraph. So, I'd be happy if somebody would check whether it works in nightly builds especially about typing multi-line comment if it's available.