Waste biomass presents both environmental challenges and opportunities for sustainable innovation. Converting biomass into biochar provides an environmentally sustainable method for waste valorization, with biochar demonstrating considerable potential for enhancing fire safety in various material applications. This study examines the fire performance of various biomass feedstocks (reed pellets, olive pits, wood chips, rubber waste) and their corresponding biochars, produced at 700 °C, using cone calorimeter tests. The biomass samples exhibited peak heat release rates (PHRR) ranging from 200 to 600 kW/m2, whereas the PHRR of their biochars was significantly lower, ranging from 20 to 35 kW/m2. Biochar derived from lignocellulosic biomass demonstrated more prolonged combustion behavior, with higher PHRR values, whereas rubber waste biochar exhibited the lowest PHRR (21.3 kW/m2) and total heat release (THR) (4.1 MJ/m2), highlighting its enhanced fire safety compared to its biomass counterpart. The composition of the parent biomass plays a crucial role in determining the biochar’s fire performance. These findings offer valuable insights into the selection of biochar for various applications, particularly as a filler in flame-retardant composites.