Hi elapid,
“Firm” is a very wide range and is also very subjective and there are also different types of firmness that different people are sensitive to so one “firm” mattress can feel very different from another one (see post #15 here). Even a mattress that has a single layer will be softer on the sleeping surface and firmer as you compress it more deeply so even without a specific “comfort layer” the upper part of the single layer will act to relieve pressure and the deeper part will act to provide support and alignment (see post #2 here).
If you used a thin egg crate topper and it worked well on another firm mattress they are very inexpensive and it may be worth considering trying something similar again to see if it helps.
Either way … you have several options that you can choose between that may work best for you and I hope you have the chance to let us know what you end up deciding to do.
Phoenix