Does this extra-firm mattress need a box spring or specific foundation?

It needs a rigid, flat, fully supportive base, and getting this wrong is the main cause of a sinking centre. Use a sturdy platform, closely spaced slats no wider than three inches, or a solid foundation under it. A weak or worn box spring lets the connected coil unit flex unevenly, creating dips within weeks and potentially voiding the warranty. The firmer the mattress, the more it relies on even support beneath.