MAYOR Bloomberg doesn’t hold a grudge. Even though John Gutfreund famously fired him from a job at Salomon Brothers many years ago, Hizzoner showed up at the legendary financier’s Fifth Avenue digs to wish him a happy 80th birthday. Warren Buffett sent a Colossal Gram from Omaha that said, “Congratulations from your youthful (79-year-old) friend.” Gutfreund beamed when his wife, Susan, sister Mary, four sons, four grand kids, and a few close pals pro duced a cake with a confection ary cigar on top, ashes and all. Then Tony Bennett sang — can you guess?