[RPG] resist damage from Fireball spell, if I have fire resistance trait

Fire resistance applies to any and all damage that’s called fire. This is described on page 197 of Player’s Handbook:

If a creature or an object has resistance to a damage type, damage of that type is halved against it.

No ifs, ands, or buts here; damage of the indicated type is halved. So if your resistance says fire, and the spells says fire, the spell does half damage. Elemental Adept defeats that protection, however.

Later in that same section, Player’s Handbook even includes an example of fire resistance explicitly combined with nonmagical resistance:

Multiple instances of resistance or vulnerability that affect the same damage type count as only one instance. For example, if a creature has resistance to fire damage as well as resistance to nonmagical damage, the damage of a nonmagical fire is reduced by half against that creature, not reduced by three-quarters.

This example would not make very much sense if fire resistance only worked for nonmagical fire, because then the fire resistance would be completely redundant.