It's pretty distinct to me. Particularly in scenarios where the suspension is actually being exercised. Compression damping over sharper jolts IMO is the most noticeable. The system switches instantaneously with turning of the knob. As AHC is always actively managing damping force over its 16 steps in response to conditions in real time, the switch is just asking it to further bias towards firmer or softer in its choices. That may be why it is harder to feel, as the system is already always actively adjusting in normal operations. The selector just biases the adjustment incrementally further one way or another.
In normal mode, the system is typically already spot on in its adjustment to conditions. I have found that in very bumpy mountain backroads, that firm setting may increase compression damping too much, such that sharp jolts mid corner can transfer too much impact energy and upset the chassis enough to loose grip. In these conditions, normal can help with keeping a faster pace. Turn in is slightly less sharp in normal which I can compensate for, but the suspension better soaks up bumps mid corner, maintaining better tire contact against the road, and keeping the chassis more composed especially as body on frame is already relatively loose with secondary motions.
Towing really heavy (1200 lb tongue), normal again already increases damping from the feedback of the accelerometers in the system, but I do find that firm can further tighten body motions. I often forget to put the system back in normal after towing and dropping off the trailer, and on the next drive I can tell its in firm by the ride alone in a couple blocks to remind me to put it back to normal.
Off-road, the system works wonders in comfort, relaxing the damping and allowing the suspension lots of travel to gently absorb big bumps. Run at higher speeds say on a fire road or wash, and the system already tightens things up. Enough for good body control while managing hits, without making it as stiff as it would behave for on-road. I may switch it to normal if I'm driving harder chasing my Raptor buddy, but that is that cars element.