Combined atomic and friction force microscopy reveals a significant modulation of atomic-scale friction related to the small periodic rumpling induced at the interface between heteroepitaxial films of KBr on NaCl(100). Transitions from dissipative atomic-scale stick slip to smooth sliding with ultralow friction are observed within the 6x6 surface unit cell of the underlying superstructure. Scanning across atomic-scale defects confirms the high-resolution capabilities of friction force microscopy close to the ultralow friction state. Strong variations of the tip-surface interaction energy across the superstructure demonstrate that subsurface chemical and size inhomogeneities dramatically change the frictional properties of the surface probed by the microscope tip.