Firm Mattresses previously had just one comfort type, but due to consumer demand different feels were introduced from Extra Firm Mattresses to Plush Firm Mattresses. There are a variety of names that describe firm mattresses, but they all fall into 3 categories:
Extra Firm mattresses - Also known as Ultra Firm or the Bedrock from the Flintstones. These mattresses are used by back sleepers who want a very firm, supportive feel. The comfort layer is a thin layer of the firmest of comfort materials. If you do not need added comfort layers to support the curves of your body due to sleeping just on your back, an extra firm mattress may be the right choice for you.
Firm Mattresses - This is a standard type that is usually described just as a firm mattress without other labels. It has some comfort materials that make the top most layer conform slightly to the body, but overall it is a firm, supportive feel. This would be a good choice for primarily back sleepers that don't need a lot of conforming to support curves.
Plush Firm Mattresses - Also known as luxury firm, medium firm, cushion firm, or padded firm, plush firm mattresses offer a compromise of firm support with just enough padding to be good for side sleepers also. Many couples compromise to choose a plush firm mattress when one partner likes a firmer feel and the other a softer feel. Medium Firm is the standard firmness of mattresses from 20+ years ago before a large amount of upholstered layers became popular.