The heteroepitaxial growth of ultrathin films of KBr on NaCl surfaces forms a square superlattice with a periodicity of 3.9 nm. This corresponds to a lattice constant ratio of nearly 6:7, which leads to a small topographic rumpling of the order of 0.1 Å. Friction force microscopy reveals significant variations of atomic friction due to these small topographic undulations of the superlattice. Transitions from dissipative atomic-scale stick-slip to smooth sliding with ultralow friction are observed within the 6x6 unit cell of the periodic superstructure caused by a variation of the energy corrugation. The experimental findings agree with numerical simulations based on an extended Tomlinson model. Strong variations of the tip-surface interaction energy across the superstructure demonstrate that chemically homogeneous surfaces can dramatically change their frictional properties due to small structural modifications. Furthermore, scanning across atomic defects confirms the high resolution capabilities of friction force microscopy close to the ultralow friction state.