[4] Firmian, became bishop of Seckau at age 31 and was also administrator of the suffragan bishopric of Trento for ten years, but the cathedral chapter largely opposed any of his proposed reforms.

In his secular sphere of power, he completed the Neue Residenz in Passau with its façade by Melchior Hefele, as well as the reclamation of parts of the Bavarian Forest (the villages of Vorder-, Mitter- and Hinterfirmiansreut(h) near the Czech border are reminders of his work).

In 1765, he bought back the Sieben künischen Dörfer, an area around Wollaberg, from Austria for 137,787 florins, which had fallen to the Habsburgs by war in 1506.

Emperor Joseph II, who was critical of the church, held him in such high esteem that he awarded him the grand cross of the Austrian Order of Sankt Stefan, and waited until Firmian's death to separate the Austrian territories from the diocese of Passau (by founding the bishoprics of Linz and Sankt Pölten).

Unlike his relative Leopold Anton Graf von Firmian in Salzburg, he strove for religious tolerance; for example, he allowed Protestants to be buried in the Catholic cemetery as early as 1777.

After Firmian's death all Austrian territories were separated from the diocese; 20 years later, the secular rule of the Passau bishops ended.