Marking/closing questions as a duplicate is a delicate subject, and there is a thin line between whether a post is a duplicate or not.

This one however hurts my eyes. While OP ctearly mentions he wants to close the latest opened window (instance) of Firefox, the question was closed as a duplicate of a question about closing a tab of Firefox. Quite different. I even explained it in the comment.

Interesting detail is that the first one to mark as a duplicate also answered the question after marking it as duplicate.

Moreover, the supposed duplicate does not offer a solution sideways as well.

Can this be undone?