We investigate the coherent-to-incoherent transition of molecular fluorescence of a chromophore above a silver surface (including bulk and thin-film systems) and explore the distance dependence of fluorescence rate enhancement. In the framework of macroscopic quantum electrodynamics, we generalize our previous theory to include multiple vibrational modes. The present theory can accurately describe quantum dynamics from the coherent limit to the incoherent limit. Moreover, we introduce a new concept Incoherent Index to quantify the degree of quantum coherence and demonstrate that the coherent-to-incoherent transition can be controlled by the dielectric environment and the molecule-silver distance. In addition, our theory indicates that strong molecule-photon (polariton) coupling can be achieved by virtue of small Huang-Rhys factors, large transition dipole moments, and appropriate dielectric material design. The present study provides a new direction for engineering light-matter interactions in polaritonic chemistry.