A sealing plug configured for sealing a passage of a curtain wall mullion. The passage has a maximum width and a length. The sealing plug includes a main body extending from a first surface to a second surface. The main body has a width at least as large as the passage maximum width and a length at least as large as the passage length. The main body is manufactured from a compressible foam. An intumescent layer is defined along at least one of the first and second surfaces.