FIRST OFF . . .

You can go home again, for a while at least. Some months ago, it had been widely rumored that Zubin Mehta, former music director of the Los Angeles Philharmonic, might be interested in coming back to his old job at the Music Center after he gave up his post with the New York Philharmonic. Mehta, whose relationship with the New York press had been less than cheery, insisted, however, that he did not want another full-time directorship. Now, Los Angeles has signed a new resident conductor, Esa-Pekka Salonen. But The Times has learned that Mehta has agreed to return to the scene of his former triumphs--and tribulations--as a guest conductor for four weeks per season, starting in 1991.