These are very common in foxes. Black debris and irritated ears (internal) are suggestive. Irritation is due to mites deep in the ear canal. They are very small but easily seen under a microscope in the laboratory. In severe cases, mites can spread to skin around the ears.
Mites should be controlled to reduce irritation and the chance of secondary infection.
Routine treatment with mineral oil is good. This treatment is better if an insecticide is included. One used for dogs and cats containing rotenone would be safe for foxes, but this requires frequent treatment. Mites are also treated with a standard dewormer such as Ivomec.