Accounting for ponderomotive force and differential Joule heating nonlinearities as well as relativistic electron mass variation, we consider the parametric coupling between a large-amplitude electromagnetic wave and nonresonant density and temperature perturbations. A nonlinear dispersion relation that is appropriate for the modulational instability is presented. Some interesting regimes for the thermal modulational instability are identified, and expressions for their increments are obtained. The results can be useful in understanding the phenomena of enhanced density and temperature fluctuations that have been observed during the artificial modification of the Earth's ionosphere by high-power radio waves.