All mattress types can be firm. Mattresses that lack pressure-relieving comfort layers (normally the top, plush cushioned layer) can feel firmer than those with these layers. This is probably why the Emma Comfort, which goes straight onto foam without a cushioning top layer, feels like a super firm mattress.
Latex mattresses are often thought of as the most firm mattresses, as well as traditional innerspring mattresses. However, with the exception of the latex Peacelily, all our picks are foam or hybrid mattresses and we rate all of them as firm. So really, we stick by our first line: all mattress types can be firm.