If the rabbit is in a good, sturdy hutch then it is relatively safe from foxes -- however, you will need to put some type of mat or flat piece of wood in the cage that the rabbit can easily get on to if it feels threatened. Raccoons, foxes, possums, coyotes, and other such critters are known for getting up underneath hutches and pulling rabbits' toenails off through the wire bottom if the rabbit doesn't have a way to protect itself. This should be in addition to any nesting box or solid "burrow box," it works best if it's something the rabbit can stand on and still reach its food and water.
I have a few articles on rabbits posted, but if you have any more questions about rabbits then please feel free to send them to me. My experience comes from my 150+-animal for-profit rabbitry, I have worked with hundreds of rabbits both indoors and outdoors. The "livestock" rabbits stay outside year-round in Wyoming, which is known for weather extremes, so I've learned a lot of tips and tricks along the way.