A firewall having a fire resistant structure, the fire resistant structure comprising: a plurality of modular metallic frame units attached together to form a substructure of the firewall; wherein each metallic frame unit has a front face and a rear face, further wherein each metallic frame unit is square or rectangular in shape so as to define an upper end face and a lower end face; a plurality of fire resistant panels (14a, 14b) arranged to clad the substructure of the firewall, wherein the front face of each metallic frame unit is covered by a respective fire resistant panel of said plurality of fire resistant panels (14a, 14b), so as to provide a fire-facing face of the substructure of the firewall, and the rear face of each metallic frame unit is covered by a respective fire resistant panel of said plurality of fire resistant panels (14a, 14b) or a non-fire resistant panel, so as to form a hollow region between said front face and said rear face; wherein the firewall comprises a first metallic frame unit of said plurality of modular metallic frame units and a second metallic frame unit of said plurality of modular metallic frame units, wherein the lower end face of the first metallic frame unit is arranged adjacent the upper end face of the second metallic frame unit, such that the first and second metallic frame units are arranged one above the other, with the first metallic frame unit uppermost; characterised in that: the firewall comprises a gasket positioned between the lower end face of said first metallic frame unit and the upper end face of said second metallic frame unit to retard heat transfer from the lower metallic frame unit to the upper metallic frame unit, wherein the gasket is configured to cover the respective end face of said first and second metallic frame units. a plurality of modular metallic frame units attached together to form a substructure of the firewall; wherein each metallic frame unit has a front face and a rear face, further wherein each metallic frame unit is square or rectangular in shape so as to define an upper end face and a lower end face; a plurality of fire resistant panels (14a, 14b) arranged to clad the substructure of the firewall, wherein the front face of each metallic frame unit is covered by a respective fire resistant panel of said plurality of fire resistant panels (14a, 14b), so as to provide a fire-facing face of the substructure of the firewall, and the rear face of each metallic frame unit is covered by a respective fire resistant panel of said plurality of fire resistant panels (14a, 14b) or a non-fire resistant panel, so as to form a hollow region between said front face and said rear face; wherein the firewall comprises a first metallic frame unit of said plurality of modular metallic frame units and a second metallic frame unit of said plurality of modular metallic frame units, wherein the lower end face of the first metallic frame unit is arranged adjacent the upper end face of the second metallic frame unit, such that the first and second metallic frame units are arranged one above the other, with the first metallic frame unit uppermost; characterised in that: the firewall comprises a gasket positioned between the lower end face of said first metallic frame unit and the upper end face of said second metallic frame unit to retard heat transfer from the lower metallic frame unit to the upper metallic frame unit, wherein the gasket is configured to cover the respective end face of said first and second metallic frame units.