LIFEMENT 6 Inch Queen Mattress, Bamboo Cover Memory Foam Bed in a Box

Which thickness should I choose - 5, 6, 8, 10 or 12 inch?

5 and 6 inch suit kids, bunk beds, daybeds and trundles. 8 inch fits teens, guest rooms and lighter adults. 10 inch is the everyday adult pick. 12 inch gives the deepest cradle for couples and side sleepers.
How firm is medium firm, and does it suit side, back and stomach sleepers?

It sits in the middle of the firmness scale. The memory foam contours to shoulders and hips for side sleepers, while the high-density support foam underneath holds back and stomach sleepers in line rather than letting them sink.
Does this mattress sleep hot?

It is built to run cool. Cooling gel and graphite infused into the memory foam draw heat away from your body instead of trapping it, so the sleep surface stays cooler through the night. A good pick for hot sleepers.
Does it work on bunk beds, platform beds, box springs and adjustable frames?

Yes to all of those, plus slatted bases, and no box spring is required. It suits guest rooms, kids rooms, dorms, daybeds and main bedrooms. Use deep-pocket fitted sheets for the 12 inch profiles.
How long does it take to expand, and is the foam certified safe?

Unroll it on your frame and allow up to 72 hours to fully expand; most loft returns within hours. The foam is CertiPUR-US certified and the cover is OEKO-TEX certified, removable.