From the article:

Such a formation, also called a cumulonimbus flammagenitus cloud, can be seen during volcanic eruptions -- or, as in this case, when a wildfire roasts the air near the ground, causing it to rise quickly.

The rising air cools and becomes a cloud, or fire cloud: a turbulent mass that can generate its own winds that worsen fires. If it becomes very large it can even generate lightning.

Hitherto, they have only really been seen in wildfire-prone Australia and North America. NASA calls them the "fire-breathing dragon of clouds".

[...]

"While this type of fire occurs regularly in Canada and Australia, it is unprecedented in France."

[...]

"We are dealing with a so-called 'convective' fire that creates its own winds -- winds that constantly shift direction, unlike a standard fire that spreads in a conical pattern. The result is that it spreads in every direction and develops multiple fire fronts, making the situation completely unpredictable.

"We don't know how the blaze will spread; the fire front is constantly shifting. We cannot fight it directly. It's a David-versus-Goliath scenario: the idea is that, at some point, we'll find a weak spot and strike there.

"Salvation will come either from the sky -- in the form of rain, though it would need to pour heavily for three days -- or by finding a way to steer the fire toward a place where it will die out on its own. The sea, for example."

[...]

"We have reached a point of 'operational impossibility' that requires us to accept the need for strategic retreat -- to understand and acknowledge a natural force beyond our control.