Hmm . . . well for me, firm is okay, since something can be firm but comfortable. The A-Class is choppy, or harsh. The Cinquecento has a better ride.
Again, it depends on what you're looking for. The Lupo has a softer ride than the Ka, so at more modest speeds, it's more comfortable. But on an undulating bit of road, it wallows, bucks, and becomes harsh, whereas the Ka retains its composure.