At the death of the Berber prince Nubel, a powerful Roman military officer as well as a wealthy Christian, his son Firmus had killed his half-brother Zammac, who had illegitimately appropriated Nubel's wealth, and become successor to his father.
Firmus has been in revolt from 372 against the comes Africae Romanus, who had been a supporter of Zammac.
The misbehavior of Romanus, who had neglected protection from African tribes to Roman cities that had refused the payment of bribes, had worsened the situation in Africa Province in 360s.
Firmus is a supporter of the Donatists against the Nicene Christians; he had ordered his followers to kill the Nicene inhabitants of Rusuccuru (present Dellys).
Firmus’s revolt against Romanus forces Valentinian to take action against both his officer and the African rebel.
When Valentinian sends his magister militum Theodosius to depose Romanus, Firmus tries to find a compromise with him, but Theodosius refuses peace to Firmus, who had proclaimed himself emperor.
With the support of the indigenous African tribes, Firmus obliges Theodosius to a bloody and hopeless campaign.
In the end, however, Firmus is betrayed by one of his supporters, and chooses suicide over capture.
After his death in 375, Valentinian issues laws against the Donatists.
Firmus’s brother Gildo, who had remained loyal to Valentinian and helped the Romans crush the revolt of his brother, is awarded with the immense patrimony confiscated from Firmus’s estate.