The chemical reaction of combustion is a complicated process, including far more than simply the initial products involved and final products produced as previously discussed.
Combustion involves many intermediate steps.
Free radicals are created and react with each other.
Some of these processes might be endothermic, but overall, the process will release more energy than in absorbs (exothermic).
These processes can be stopped using extinguishing agents such as Halons. These interrupt the intermediate reactions and inhibit combustion.