This isn't a duplicate of T3669. This discusses the case where you've begun typing a comment, but have not hit "Ready" yet, so the page still has an editable text area on it.

T3669 discusses a case slightly later, where you've hit "Ready" and turned the text into a draft, but haven't hit "Submit" to actually publish the comment.

The remedies for these issues may be substantially different. Notably, this case is probably resolved cleanly with an onbeforeunload handler, even in Firefox.