Guilty. Joseph Bremer’s medical report(s) indicate that he was suffering from an iron deficiency anemia. He did not know that, as the analysis result did not arrive until after his death. An anemia patient can lose consciousness in case of an insufficient oxygen concentration in the air.
All this indicates that Joseph, being sensitive to the oxygen concentration, fainted during the fire, remained unconscious and died.
All the characters of this story had different motives for this crime, but only Joseph had no alibi and apparently tried to set his own house on fire to claim the insurance money so he could pay his debts. He underestimated the severity of his illness, lost conscious and subsequently died in the fire.
So, the fire victim is the firestarter.