Put the tofu in a medium saucepan and add 5 cups water and the salt. Bring the water to a boil and then cook gently over medium heat for 5 minutes, until the tofu is soft. Turn the heat off and let the tofu sit in the water.
Heat a wok over high heat until a droplet of water sizzles and evaporates immediately upon contact. Swirl the vegetable oil around the bottom and sides of the wok to coat it evenly. Add the ground beef to the wok and stir-fry for about 3 minutes or until the beef has browned slightly. Remove the beef from the wok and transfer it to a bowl.