Concepts of cavity QED such as strong coupling or the Purcell effect are typically derived for two-level systems. These concepts can be translated to the more complex case of molecules where internal electronic transitions are inherently coupled to many molecular vibrational modes via Holstein-type interactions. We develop a quantum Langevin equations approach that provides analytical insight into the cavity-modified radiative emission branching ratio of a single molecule, the strong coupling regime with molecules and Frster resonance energy transfer between donor-acceptor molecules.