Path integral Monte Carlo simulations are used to study quantum Lennard-Jones solids and to examine the dependence of the melting temperature on the degree of quantum delocalization. To determine the solid-liquid coexistence conditions and metastability limits, an umbrella sampling strategy is adopted to generate Landau free energy curves. Simulation results for the melting point dependence on the particle mass are shown to be reasonable in comparison with experimental data on the melting curves of solid H-2 and D-2. Both the enthalpy and entropy of melting are shown to decrease with increasing strength of quantum effects. For solid molecular hydrogen and its isotopomers, zero-point energy effects are shown to be important even at melting temperatures.