This work presents a theory of the frequency-resolved light emission of active two-dimensional dielectric microresonators, which are characterized by a highly nonparaxial mode structure and frequently feature a position-dependent dielectric constant and nonuniform gain. The Lorentzian intensity profile is characterized by an appropriately generalized Petermann factor, a renormalized peak position, and the cold-cavity resonance lifetime Gamma. The theory also delivers a relation of Gamma to the laser threshold that improves earlier phenomenological expressions even for the case of a homogeneous medium.