BUENOS AIRES, May 19— Mario Firmenich, the best-known leader of the leftist guerrillas who fought the Argentine Government in the early and middle 1970's, was found guilty today in a 1974 kidnapping-murder case and sentenced to life in prison.
But under the terms of an agreement with Brazil, from which he was extradited in 1984, the 39-year-old Mr. Firmenich will serve no more than 30 years.
Judge Carlos Luft of the Federal Court in the Buenos Aires suburb of San Martin, held that Mr. Firmenich was the mastermind of the kidnapping-for-ransom of Jorge and Juan Born, brothers who had acquired immense wealth as grain traders.
The kidnappers killed the Borns' driver and a business associate who was with them when they were abducted on Sept. 19, 1974. The Borns were released months later after payment of an estimated $60 million in ransom.
Mr. Firmenich was the principal leader of the Montoneros, the largest of the Argentine guerrilla organizations, whose actions included bombings, kidnappings and assaults on police and military units.
The prosecution of Mr. Firmenich was part of a policy announced by President Raul Alfonsin when he took office in December 1983 to bring to justice both the guerrilla leadership and the military leaders who took power in 1976 and oversaw the counterinsurgency campaign in which an estimated 9,000 people died or disappeared.
Mr. Alfonsin ordered the prosecution of seven people said to be former guerrilla chieftains and of the nine former armed forces commanders who made up the three military juntas that ruled Argentina between 1976 and 1983.
Of the accused guerrilla leaders, Mr. Firmenich is the second to be found and tried. The first, Ricardo Obregon Cano, a former governor of the province of Cordoba, was found guilty of ''illegal association'' with the Montoneros and is serving a six-year sentence.
All nine former military commanders were tried on charges of responsibility for deaths, disappearances, torture and other abuses. Five were convicted and are serving sentences ranging to life in prison.
The Government's campaign against the former military and guerrilla leaders is distinct from the court proceedings initiated by human rights groups and prosecutors against several hundred other military and police officials, some of whom would go free under legislation now pending in Congress. Trials are also under way in at least five other cases involving guerrilla activity.
