* Ferrets belong to the order Carnivora, suborder Caniformia (dog-like carnivores).
* Domestic cats belong to the order Carnivora, suborder Feliformia (cat-like carnivores).
This means the divergence between ferrets and domestic cats happened a very long time ago, before the split between the dog-like and cat-like carnivores.
Estimated divergence times:
* Caniformia and Feliformia: Around 40-50 million years ago
* Mustelids (family of ferrets) and Felidae (family of cats): Even further back, within the Caniformia and Feliformia branches.
So, the divergence is at a very high taxonomic level - much higher than, for example, the level of species or genus.