They can easily jump the fences.
I haven't seen a fox jump a fence. They can go through gates, but if you lock them for all pets it seems to keep them out. The wild foxes on my lot prowl around the outside of the fence, and crouch down and growl, but they haven't gotten in since I locked the gate.
The lock feature seems to be a bit glitchy, though. I want foxes locked out and chickens locked in, but the first time I applied these settings to my current lot the chickens kept wandering all over the place and when I clicked lock for chickens again, a fox got in and stole one of their eggs. Now I have it locked for everyone and allowed access to only the household, and that seems to be working. On the last lot I played, locking the gate for pets and chickens worked fine.
