We studied the flame propagation and combustion properties of a lightweight fibrous foam produced from a layered double hydroxides (LDH) modified thermomechanical pulp fibres. The in situ synthesis of Mg-Al LDH with pulp fibres was engineered to include both micron and nano-sized particles. The method allowed loading the fibres with LDH up to 34% (w/w). Observed pyrolytic effects included 60% reduction in CO₂ production rate, and similar reductions in peak heat release rate (PHRR) and in amount of soot during the oxidative pyrolysis. The in situ synthesised LDH particles shielded the fibres from external heat by reducing the rate of oxidation and liberation of volatile gases. Effective charring was observed at the interphase of LDH nanoparticles and organic material.