The flammability of polypropylene (PP) and recycled PP composite systems was thoroughly investigated by using cone calorimetry and thermogravimetry techniques in order to quantify the effect of brominated and chlorinated fire-retardant systems. In this study, the efficiency of the fire-retarding systems was defined from the relation between the amount of fire retardants and the characteristic quantities in cone calorimetry (M-HRR, THR, and M-EHC) and the limited oxygen index. The brominated fire retardants showed fire-retarding capability superior to that of the chlorinated system, and the recycled polymer was thermo-oxidatively stabler than the pristine PP systems seemingly because of the fillers and blended polymers. Comparing the degradation behaviors of the composite systems in nitrogen and air environments, the halogen compounds retarded the decomposition rates in both cases, deactivating the reactive radicals of the polymer and restricting the flammable gases from being diffused into the unreacted polymer. Overall, the optimal amounts of fire retardants in different polymeric systems could be evaluated and correlated with several fire indicators in a relatively simple manner.