A removable firebox for use in a portable, collapsible barbecue grill including retractable heating grate on top of the firebox and a retractable mid-level cooking grill for providing high-temperature cooking. A lower main grill is disposed in a base cavity of the barbecue grill for providing moderate temperature cooking. The firebox rests in the base cavity of the grill housing and provides heat to a cooking area defined by the base cavity, an upper lid of the housing and a pair of side walls extending upward from the base cavity. The firebox and/or the upper lid includes insulation and/or cooling baffles to protect users from heated surfaces. Venting is provided from the sides and rear of the grill housing. When collapsed, the grate and grills fold onto or are retracted within the firebox, and the entire firebox assembly is stowed in a sealed grill housing. The top and bottom portions of the grill housing mate to form a sealed container-like structure for convenient transportability or storage. The barbecue grill also includes foldable legs which lock into extend position, when deployed.