That depends on the fire.
Magic fire does not work like regular fire in all ways. You can't dispel a normal fire or put it out by punching a concentrating wizard in the face.
In 3e conjuration magic could summon some actual fire from an existing fire or the elemental plane of fire. It would not be subject to spell resistance or dispelling after the spell was completed because it was actual fire. 3e evocation magic could do magical fire effects too but they would be subject to SR and dispelling if they were ongoing (like a wall of fire).
I thought it was really cool in 4e that fireball spheres were grid cubes. It showed how magic tapped into mythos style non-euclidean geometry.