Because mattress manufacturers often use several model numbers for the same mattress, it's impossible to know which specific mattress is the firmest. Additionally, manufacturers define "firm" for themselves. Lack of an industry standard makes companies' firmness ratings inconsistent.
Mattress manufacturers often make several versions of the same mattress, changing only superficial features, such as color, and assigning a different model number to each version. The model numbers are specific to the retailers that sell them, not to the mattresses themselves.
Further complicating comparisons is the lack of an industry standard for mattress ratings. Consumers should test mattresses in a mattress store before buying one. Good Housekeeping recommends lying on each one for at least 15 minutes before making a purchase decision.