An insert adapted to be inserted between a foundation of a bed and a mattress of the bed. The insert includes a frame having a first side rail and a second side rail spaced apart from the first side rail. A plurality of elongate upper slats extend between the first side rail and the second side rail of the frame at a first level with respect to the frame and are spaced apart from one another. A plurality of elongate lower slats extend between the first side rail and the second side rail of the frame at a second level located below the first level of the upper slats. Each lower slat is located below a gap formed between a pair of adjacent upper slats. A mounting member resiliently supports the ends of two adjacent upper slats and rigidly supports the end of a lower slat that is located below the gap formed between the two adjacent upper slats, whereby the firmness of a mattress overlying the insert can be selectively adjusted.