Assuming that the tennis balls are identical in terms of size, shape, and mass, and neglecting air resistance, both tennis balls would fall down at the same rate regardless of whether one of them has been exposed to fire or not.
The reason for this is that in the absence of air resistance, the only force acting on the tennis balls is gravity. The force of gravity depends solely on the mass of the objects and is independent of their composition or temperature. As a result, both tennis balls would experience the same gravitational force, causing them to accelerate toward the ground at the same rate.
It's important to note that if one of the tennis balls were to burn, it might experience changes in its physical properties such as weight or shape, which could potentially affect its motion. However, if we assume that the tennis ball has been heated but not yet burnt, there would be no significant alteration in its mass or shape that would affect its fall.