The aim of this work was to verify the influence of graphite and wood-based fillers on the flammability of flexible polyurethane foams (FPF). Expandable graphite (EG) and cellulose (C) fillers were added to FPFs to improve their thermal stability and reduce their flammability. Four types of foams have been compared: FPF, FPF with the addition of EG, FPF with the addition of C and FPF with the addition of both fillers. Linear flammability tests and pyrolysis combustion flow calorimetry (PCFC) were performed to assess the flammability of these materials. It was found that the addition of cellulose does not improve the fire reaction, but a combination of both the EG and C fillers mixed together was able to achieve a small reduction in flammability, as confirmed by a linear flammability test and PCFC. The best properties observed by PCFC were from FPFs with EG. Usage of cellulose filler separately is not a good method for the assessment of higher thermal stability and lower flammability of FPFs. Thermal properties were measured by thermogravimetric analysis and dynamic mechanical analysis. These results showed that especially EG addition allows to achieve a positive effect on the thermal stability of the tested materials. Mechanical and physical tests (density, hardness, flexibility and irreversible strain) showed that the presence of graphite or cellulose filler results in changes in the properties of the FPFs, but these changes are not extensive. Fourier transform infrared spectroscopy analysis showed that only small changes exist in the chemical structure with the addition of the fillers. The introduction of EG and EG+C fillers into an FPF may reduce its flammability.