But as said above mobs will not willingly walk off such an edge.
They will if they see a miner on the other side of the hole and have no other way to get to the other side. Hostile mobs, at least. Passive ones will just occasionally jump off a cliff.
Often the hostiles won't make that leap. In fact, I had a spider that wouldn't drop into a 2-deep hole when I was on the other side and up 1. But then, I've also had one repeatedly climb a hill away from me then jump back down trying to reach me.
If the just happen to be jumping around, they can in fact fall off, just as any other mob will do. Also, b/c mobs have the same amount of health as you, judge how far it takes for you to die and compare it to any situation.
While chasing, mobs sometimes will jump off, but sometimes they just stop. I've noticed this a lot with creepers and skeles.