A protective helmet includes an outer shell having a generally dome-shaped section and a brim extending outwardly from the dome-shaped section. The dome-shaped section and the brim are formed from a thermoplastic material. The helmet further includes a brim support in operative connection with at least a portion of the brim. The brim support is formed from a material that is more heat resistant than the thermoplastic material. The brim support is sufficiently rigid to limit deformation of the brim upon exposure to high temperature. The brim support material can, for example, have a melting point above 500° F. A method of limiting high-temperature deformation in a helmet includes molding an outer shell of the helmet to consist essentially of thermoplastic material, the outer shell comprising a generally dome-shaped portion and a brim extending outwardly from the dome-shaped section by at least 2.5 inches at the rear of the helmet; and operatively connecting a rigid brim support to the brim, the brim support being formed from a material that is more heat resistant than the thermoplastic material.