The fires were either ignited by carelessly disposing of fireworks and packaging – or “more alarmingly had been started deliberately”, a spokesperson said.
The service said members of the public could have been put at risk when firework boxes were set on fire and left in parks on Mill Road in Kettering and Briar Hill Playing Fields in Northampton.
Mr Sadler said: “These fires have potentially taken resources and crews away from other incidents and people that needed our help.
“Fireworks are live explosives that can cause serious harm and damage when used incorrectly, and it is concerning that we have seen incidents where people have set fire to them in a dangerous manner deliberately.
“While we want people to have fun, we really want to urge members of the public that if they are going to have their own fireworks display, to please follow the manufacturer’s instructions and to dispose of them carefully and responsibly.”