Lignin and glyoxal are potential sustainable substitutes for preparing lignin-based epoxy resins but exhibit poor flammability; although lignin incorporation enhances the charring capacity of thermosetting materials, intense burning occurs on ignition. A metal-organic framework (P-MOF) comprising cobalt and phosphorus/nitrogen components was therefore synthesized to improve fire safety. Limiting oxygen index (LOI) and vertical burning tests (UL-94) indicate excellent flame retardancy, with a higher LOI value and V-0 rating level achieved after P-MOF addition. Cone calorimeter tests investigating gaseous inhibition of epoxy matrixes reveal that heat release and smoke production dramatically decrease on addition of P-MOF because of fire quenching (phosphorus organic component) and smoke suppression (cobalt component). Synergy between lignin and P-MOF components suppressed combustion of condensed phases, producing more char residues with a higher degree of graphitization. A mechanism for the flame retardancy of the epoxy composite is proposed, which highlights the promise of P-MOF additives for reducing fire hazards due to cobalt-catalyzed CO oxidation during epoxy burning.