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.
The Superior Planets are most efficacious; the inferiour are weak, and easily do suffer by the Superiours, and receive good or ill from them. - William Lilly (1602.-1681.)