In this work, we aim to understand the firing behaviour of three representative thick-film resistor compositions used in force and pressure sensors. The dependence of the materials' microstructure and properties (sheet resistance and its temperature coefficient, gauge factor) is studied as a function of firing temperature and time, and cooling rate (furnace or quench). The stability of the properties is assessed by annealing at intermediate temperatures (100 and 250 °C). Microscopic and structural analysis is also carried out. The results are discussed in the light of the possible evolution mechanisms of the resistor materials: diffusion, dissolution, precipitation and stress relaxation.