His maternal uncle Count Johann Ernst von Thun was Bishop of Seckau from 1679 until 1687 and Prince-Archbishop of Salzburg from 1687 to 1709.

His nephew, Karl Joseph von Firmian, the Austrian plenipotentiary minister in Milan, was renowned as a patron of the arts, including poets such as Giuseppe Parini, musicians such as Johann Ernst Eberlin and painters such as Giambettino Cignaroli.

Firmian had prepared for an ecclesiastical career, received his ordination in Rome in 1707 and became provost of the Salzburg chapter in 1713.

[2] After the expulsion of the Protestants, Firmian divided the Salzburg territory into four mission areas: Augustinian, Capuchin, Benedictine and Franciscan.

Archbishop Firmian is buried at the crypt of Salzburg Cathedral while his heart rests under the floor of his 'dearly beloved' Schloss Leopoldskron chapel.