Foxes don't jump over 2 blocks high or taller walls, even with a target like a chicken on the other side.
Foxes are techically able to jump 3-4 blocks high, but they only jump over max 1.5 block height walls.
They have an easier time jumping over 1.5 block height fences than 1.5 block height walls made of slabs.
They sometimes manage to "accidentally" jump over 2 block high walls, when there's no wall blocking the way or if they are trying to jump over a lower height one. (As you can see in the attachment screenshot)