There is really no good answer to your question because "firmness" is totally subjective. I think what would help you most is to understand your anatomy thoroughly.
The testicles contain the testes, where sperm are produced, and the epididymus, where sperm are then stored. There are lots of tubes, etc. inside, all coiled up. These can feel squishy or firm, depending on the circumstances.
I suggest you read up on these fascinating structures. Here's some good information for you:
The epididymis is a small organ that sits on top of the testicle. It is made up of a little bundle of squiggly tubes and is a "holding pen" for sperm. When a man ejaculates, sperm are moved from the epididymis into the vas deferens, the tubes that take sperm up and out of the scrotum area and into other ducts so they can be ejaculated as a part of semen. Sperm that don't get ejaculated live in the epididymis for around four to six weeks before they die and are reasborbed into the body. It's very similar to the way old blood cells die and are reabsorbed and recycled. This is why it's impossible - even if you never ejaculated in your whole life - to have "excess" sperm build up in your body.
The epididymis and testicle are the two major organs inside the scrotum. They don't just float around in there, though. They are anchored to the body by what is called the spermatic cord. The spermatic cord is a bundle of nerves and blood vessels that serve the testicle and epidydimis, plus the vas deferens, is a long tube which carries sperm up and out of the scrotum so that it can be ejaculated.
As long as you aren't in pain and have no symptoms or discomfort, it's likely that your testicles are just fine. Dr. J