Someone mentioned Princess Sophie Charlotte of Bavaria, later Duchesse d'Alencon, as another royal relative of Mathilde who died as a result of fire. Though not a contemporary of Mathilde, one might also mention Archduke Alexander Leopold (1772-1795), the fourth son of Emperor Leopold II of Austria, who died of accidental burns acquired while arranging a fireworks display at Laxenburg Castle at the age of 23.
Perhaps the truly lethal thing about accidental fires back in those times was the lack of readily available fire extinguishing equipment, as well as lack of appropriate medical attention, more so than flammable clothing.