Futon mattresses are usually firmer than bed-only mattresses. These mattresses need to be thin to serve as both a sofa and a bed. However, the overall firmness depends on the mattress, and also the type of frame that you have.
Regular foam futon mattresses are the firmest option, followed by memory foam products, which do a better job of conforming to your body. Coil futon mattresses can be soft or firm depending on the tension of the coils.
Some futons have firm cushions, but the mattress gets covered by additional pillows when it is in the sofa position. These can make the futon seem soft when you sit on it, but since you won’t use the sofa-specific padding when the futon is in the sleeping position, it will be more firm.