What foundation does this high-density foam mattress need? Can it go on an adjustable base?

It needs a flat, solid, fully supportive surface and is not rated for adjustable bases. Use a platform bed, a solid foundation, or a bunkie board, with any slats spaced under 3 inches apart so the 6" foam cannot bow between them. A box spring is not suitable, since its give would let this thin foam profile flex unevenly. Continuous firm support is what keeps the surface level and the feel firm.