What's all that white fluff drifting around France?

If you've been walking around certain areas of France this week, you may have noticed that the air is filled with drifting balls of white fluff. But what is it?
If you've been walking around certain areas of France this week, you may have noticed that the air is filled with drifting balls of white fluff. But what is it?

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What are those white, fluffy objects floating around?