If there was any aggression from the shrimp and he did any damage to the fish I wouldnt doubt it. Plus the fish was new to the tank and probably wasnt adjusted yet and then he gets picked on. Not a good start in a new tank. My firefish is still super skittish around me and its been months and months lol.
Just guessing by the way, theres really no way to know for certain what happened other than the firefish didnt make it. :-(