Hi Jae,
There is no standard definition of firmness that is consistent between manufacturers and the firmness or softness of a mattress is also very subjective depending on the type of firmness or softness that you are most sensitive to (see post #15 here).
There is also no consistency between the firmness ratings of different manufacturers and one manufacturer’s firmness rating of medium may be rated as firm by another manufacturer. There is no consensus in other words about what the different “word ratings” really mean.
The perception of firmness and softness is also relative to each person’s body type, sleeping positions, individual sensitivity, and preferences.
There are also different specs that can contribute to the sensation of softness or firmness (see post #4 here) and the interaction between them can be difficult to predict because they can be different for different people.
For example you may need softer comfort layers for pressure relief for your fibromyalgia and firmer support layers for good alignment.
Because there are so many unknowns, variables, and preferences involved … it’s not really possible or someone else to know for certain which mattress you would describe as firm or soft or would be best for you. The most effective way to decide which mattress is the best “match” for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) would be your own personal testing or experience hopefully using the testing guidelines that are linked in the tutorial post.
If you can’t test a mattress in person then your best odds of success would be a more detailed conversation with a manufacturer or retailer where you can provide them with more information about you and your experiences on different mattresses that you have tested or used that they are familiar with so they can use the information you provide to give you some “best efforts” guidance about which of the mattresses they make or sell would most likely be the best match for you (see mattress firmness/comfort levels in post #2 here).
Regardless of which mattress you end up choosing … I would make sure that you know the quality of the materials inside it (see post #4 here) so that you can make sure that your mattress doesn’t use lower quality/density materials or have any “weak links” in the design that can soften much more quickly than higher quality materials and result in the premature loss of comfort or support.
Phoenix