The first broomstick Harry had was called the Nimbus Two Thousand. But at one Quidditch match at Hogwarts in Harry's third year, Harry got attacked by Dementors and he went unconcious and fell off his broom. Then the broom flew away into the whomping willow... And that tree doesn't like being hit. So as Harry's first broom got smashed to bits, his godfather, Sirius, sent him a Firebolt