For the first three years of Harry's Hogwarts life, we see a new broom coming out as the fastest each year:
Several boys of about Harry’s age had their noses pressed against a window with broomsticks in it. “Look,” Harry heard one of them say, “the new Nimbus Two Thousand — fastest ever —”
-Harry Potter and the Philosopher's Stone, Chapter 5
“Very latest model. Only came out last month,” said Flint carelessly, flicking a speck of dust from the end of his own. “I believe [the Nimbus Two Thousand and One] outstrips the old Two Thousand series by a considerable amount.
-Harry Potter and the Chamber of Secrets, Chapter 7
“Just come out — prototype —” a square-jawed wizard was telling his companion.
“[The Firebolt is] the fastest broom in the world, isn’t it, Dad?” squeaked a boy younger than Harry, who was swinging off his father’s arm. “Irish International Side’s just put in an order for seven of these beauties!” the proprietor of the shop told the crowd. “And they’re favorites for the World Cup!” -Harry Potter and the Prisoner of Azkaban, Chapter 4
There's no mention of a faster broom in any of the remaining books.
So, did the broomstick industry just stop their research after the Firebolt came out? Or were they unable to make anything better than the Firebolt?