1. Safety concerns: The Firebolt was a highly advanced broomstick, capable of speeds and maneuvers that even experienced wizards like McGonagall found unnerving. She worried that Harry, being only 13 years old and still learning, was not yet mature or experienced enough to handle such a powerful broom safely.
2. Suspicious origin: The Firebolt was given to Harry anonymously. While it was ultimately confirmed as a gift from Sirius Black, McGonagall was initially unaware of its source and concerned about the possibility of it being a dangerous object or a means of espionage.
Even though she eventually returned the broom, McGonagall's actions were driven by a strong sense of responsibility and a desire to protect Harry, even if her methods seemed harsh at the time.