North Yorkshire Fire Service said crews from Acomb and Huntington had dealt with a shed fire at a property off Poppleton Road shortly before 8.40pm on Saturday.
A spokesman said: "Fire and heat damage was caused to the contents of the shed and slight smoke damage to the shed roof. The cause is believed to be a rogue firework."
Firefighters from York had already been called to a home in Beckfield Lane, where a conifer tree was on fire.
The crew found the fire was already out on arrival but carried out an inspection.
It is believed the cause was a firework, the spokesman said.