To set the firemode of the current weapon of a player, simply use the workaround described by Killzone_Kid in the first note of the forceWeaponFire wiki entry:
_weapon = currentWeapon player;
_ammo = player ammo _weapon;
player setAmmo [_weapon, 0];
player forceWeaponFire [_weapon, "FullAuto"];
player setAmmo [_weapon, _ammo];