A coffin in a dream rarely predicts death literally -- it more often marks an ending you're processing, or feelings you've buried rather than let yourself fully grieve. An open coffin can point to something from the past resurfacing, while a sealed one often reflects a chapter you've genuinely closed.
Mars of a nativity, and not joined to good planets, signifieth the native to suffer by envy and hatred. - William Lilly (1602.-1681.)