I want to clarify that ability score modifications *include* the ability adjustments you get from the base class used when creating the subrace character. So the net gains, or losses, are in addition to the base race modifications. This means that after character creation, the following are the net gains or losses applied to each subrace character:
Gnoll (Base Race of Half-Orc): +2 Strength, +2 Constitution
Orc (Base Race of Half-Orc): +2 Strength, -2 Wisdom
Goblin (Base Race of Halfling): -2 Charisma
Tiefling (Base Race of Human): +2 Dexterity, +2 Intelligence, -2 Charisma
Drow (Base Race of Elf): +2 Intelligence, +2 Charisma
Fire Genasi (Base Race of Human): +2 Intelligence, -2 Charisma
Gnoll (Base Race of Half-Orc): +2 Strength, +2 Constitution
Orc (Base Race of Half-Orc): +2 Strength, -2 Wisdom
Goblin (Base Race of Halfling): -2 Charisma
Tiefling (Base Race of Human): +2 Dexterity, +2 Intelligence, -2 Charisma
Drow (Base Race of Elf): +2 Intelligence, +2 Charisma
Fire Genasi (Base Race of Human): +2 Intelligence, -2 Charisma