It wouldn't be. Only avgas is leaded. Avgas is only used in piston engines. Turboprops and jet engines use jet A. Jet A is kerosene.
Lead is only in avgas used by piston engines. Approximately zero firefighting aircraft use piston engines at this point - turbine power and reliability are so good that most firefighting aircraft that started with piston engines have been retrofitted (e.g. Calfire's Turbo Trackers).
Why would there be any interaction or correlation between aviation fuel and fire retardant?
Fire retardant is typically delivered via air. If the aircraft is running on avgas, the retardant may mix with the exhaust on release.