Municipal firefighters count on their protective garments to avoid skin burns caused by thermal and flame exposures. Typical firefighting garment consists of three layers of different fire-resistant fabrics named as outer shell, moisture barrier and thermal liner. This paper employed a numerical heat transfer model for firefighters’ garments, which paid more attention to modeling air gaps bounded between garment’s layers. The paper explored and compared the influences of air gaps bounded between garment’s layers on its protective performance. Specifically, the paper investigated the effect of a variation in the air gaps between the garment layers from 1 mm to 6 mm, a variation in the backside emissivity of the outer shell and moisture barrier layers from 0.9 to 0.1 and a variation in their typical thicknesses from 50% to 200% on the protective performance of garment. The results showed that increasing the width of the gap between the moisture barrier and the thermal liner, reducing the outer shell backside emissivity and increasing the moisture barrier thickness improves the protective performance of firefighters’ garments more than does increasing the width of the gap between the moisture barrier and outer shell, reducing the moisture barrier backside emissivity and increasing the outer shell thickness, respectively.