The siege of Firmum (90 BC) saw Pompey Strabo besieged in the city after suffering a defeat at Mount Falernus, delaying his attempt to besiege Asculum (Italian Social War).
The Social War began with a massacre of every Roman found in the city of Asculum, in the centre of southern Picenum, on the Adriatic coast of Italy. The capture of Asculum immediately became a major Roman objective, and Gnaeus Pompeius Strabo (father of Pompey the Great, the triumvir), who was a major landowner in Picenum, was sent to besiege the place. The Italians were determined to hold onto the place, and three of their generals (C. Vidacilius, T Lafrenius and P. Ventidus or P. Vettius Scato) united their armies and defeated Pompeius at Mount Falernus (probably somewhere to the north of Asculum). Pompeius was forced to flee east to Firmum (modern Fermo), a hilltop Latin colony, about four miles from the coast, and just south of the river that probably ran past Mount Falernus.
Lafrenius was left to besiege Firmum, while the other Italian commanders moved off to other theatres. Pompeius rearmed the survivors of the defeat at Mount Falernus, but at first was unwilling to risk a battle. He changed his mind when he learnt that another army was approaching the city. This is sometimes said to have been a Roman relief army, in which case Pompeius's decision to attack would have been an attempt to prevent a political rival from sharing in the victory, but it is more likely to have been Italian reinforcements, in which case Pompeius decided to attack before it was too late. The attacking forces were clearly not powerful enough to conduct a full blockade of the city, as Pompey's attack would prove, but that would have changed if reinforcements could arrive.
Pompeius decided to carry out a two pronged assault on Lafrenius. He would lead a frontal assault, while his legate Sulpicius was sent out to attack Lafrenius in the rear (suggesting that some parts of the town weren't being very closely watched).
Pompeius and Lafrenius were soon engaged in a fierce battle, which lasted until Sulpicius set fire to the Italian camp. The Italians also suffered the loss of their general, as Lafrenius was killed in the battle. They fled south to Asculum, followed by Pompeius, who was finally able to besiege the place.
This may be the same battle mentioned in Livy in which Pompeius routed and besieged the Picentes, referring to the victory outside Firmum and the siege of Asculum. In the aftermath of this victory the Roman magistrates resumed the use of their purple bordered togas, which they had renounced at the start of the war.