tl;dr: French firefighters are encountering a pyrocumulonimbus—a "fire cloud" that generates its own weather including lightning and shifting winds—for the first time, a phenomenon previously seen mainly in Australia and North America. The convective fire creates unpredictable, multi-directional fronts that cannot be fought directly, forcing crews into a defensive posture and hoping for heavy rain or for the fire to burn out at a natural barrier like the sea.