HAVANA, Cuba, May 5 (AP) --Harvey S. Firestone III, 32. of the wealthy tire manufacturing family, plunged to his death Thursday night from the 20th floor of the Havana Hilton hotel. Cuban officials ruled his death a suicide.

A spastic, Firestone had been crippled from birth and was confined to a wheelchair.

Havana police said a cousin, David M. Firestone, 29, of Sarasota, Fla., told them the young heir had tried to commit suicide two months ago in Florida.

Lands on Roof

Police gave this account:

Firestone had been lying on a couch while his attendant, William J. Didas of St. Petersburg, Fla., was taking a bath. Clad only in white shorts, Firestone crawled to the balcony of his suite and from a chair top either fell or jumped over a 3 ? foot aluminum grill barrier.

He struck a third floor parapet above the main entrance to the 25 story hotel in the heart of the Cuban capital's fashionable Vedado district.

Grandson of Founder

The victim was the only son of Harvey S. Firestone Jr. of Akron, chairman of the board and chief executive officer of the Firestone Tire and Rubber company and the son of the founder of the company.

The younger Firestone made his home in St. Petersburg.

Havana police said Harvey and David firestone and Didas registered at the hotel Thursday and said they planned to stay three days.

Graduated in Law

Altho unable to write because of his infirmity, Firestone, with the aid of a tutor, was graduated magna cum laude from the University of Miami and finished third in his class at the Stetson university law school in 1959.

The tutor, Everett E. Cushman of Gulfport, Fla., said he and Firestone had planned to open a law office together.

Firestone married Beverly Lou McFarlan of Brecksville, O., July 21, 1956, and their daughter, Diane Elizabeth, was born Sept. 15. 1958.

He is also survived by his mother, Mrs. Elizabeth Parke Firestone, and three sisters, Mrs. William C. Ford of Detroit, wife of the vice president of the Ford Motor company, and Mrs. Charles S. Willis Jr., and Mrs. John E. Ball, both of New York City.

Firestone's grandfather was Guy J. Parke, Decatur, Ill., business man.

HAVANA, May 6 (AP) -- A Cuban judge Friday ruled as suicide the death of Harvey S. Firestone III, heir of the wealthy rubber family who plunged from the 20th floor of his hotel suite. He had been crippled since birth with cerebral palsy.

A cousin said the 30 year old Firestone tried to jump from a speeding car in Florida two months ago. He was a lawyer and resident of St. Petersburg.

Firestone arrived at the Havana Hilton hotel Thursday with his cousin, David Firestone, 29, of Sarasota, Fla., and an attendant, William J. Didas of St. Petersburg. They had left him alone in his room.

Firestone leaves his wife, the former Beverly Lou Mc-Farlan of Becksville, O., and a 19 month old daughter, Diane Elizabeth. He was a grandson of the founder of the Firestone Rubber company and a son of Harvey S. Firestone Jr., present head of the company.