firm tofu goes a bit softer when you cook it.
Get baked tofu, it's avaialbe at any Thai supermarket or Korean maybe. It is harder, denser and is really chewy and thick, like a fat, thick dense steak texture, GOD I LOVE IT@@!!!!!
It is pressed dow with weight, and infused with spices and marinade when they make it- you can get it plain ( not infused ) too, = it is thinner than the usual slab tufu in plastic boxes.
It is very good, very thick and chewy dense, you can bite into it- it has the same hardness as a steak or a piece of cheese.
I adore that stuff.......I find normal plastic tub tofo really unpleasant, yet its the most commonly found, probably because it can keep for weeks in that water.
If you want nice tofu, find a good Thai market, or Chinese, Korean etc, they will have lots of different types and fresh made- it is dry, and not sold in a plstic tub filled with water.
If you are lucky tou'll find really fresh hot tofu, made that morning0 it tastes SO amazing, like peanutty, fresh bread, and the texture is like fresh white bread.