jmc

Because we have $3^2+4^2=5^2$, the base of the pyramid is a right triangle with legs of lengths 3 and 4. Therefore, the area of the base is $3\cdot 4/2=6$ square feet. The altitude of the pyramid is 6 feet. The volume of the pyramid is one-third the product of the area of the base and the altitude, which is $6\cdot 6/3=\boxed{12}$ cubic feet.