Neither $575 tickets nor three companies of noisy firefighters spoiled the opening gala over the weekend of the San Francisco Opera. The company's performance of Rossini's "Il Barbiere di Siviglia" drew about 3,000 people to the War Memorial Opera House. The firefighters arrived after a faulty smoke detector--activated by equipment warming food for a post-opera supper--went off at the nearby Veterans Memorial Building. After the alarm, action-garbed firefighters on the run with walkie-talkies and hoses mingled with the arriving glitterati. The show did go on, however.