To be precisse, Mamoudou Gassama hasn’t been given citizenship. He has been given a residence permit, which will enable him to get a job and apply for citizenship. He hasn;t been given a job with the fire brigade either, but rather an internship.
And it turns out that there is a specific legal provision for granting citizenship to someone who “has performed exceptional services for France, or whose naturalisation would be of exceptional interest for France”.