It depends on which country you want to fly in firefighting duties. In Canada you need a certified mountain course to fly in British Columbia on fires, but it's not written as an endorsement on your pilot's licence. In Europe, only Spain has a specific endorsement (80 hours groundschool and 15 hours flying time to have it stamped on your licence). Chile also has a similar requirement...
So if you give us some more info about the country you want to fly in and the licence you have, there can be more information available.
Regards