I have slept on two types of futons: 100% cotton and cotton layered with foam rubber. The all-cotton futon will be very firm from the start and will compact further with use. The cotton and foam version's comfort is directly related to the amount of foam rubber used in the mattress' construction. I found the combination futon to be comfortable while the all-cotton was too firm for me.
I was able to make my futons more comfortable by adding a feather bed or a pillow-top mattress pad.
It is important to be sure that the foam rubber used in your futon is not synthetic. Synthetic foam rubber can and usually does contain toxic chemicals. The chemicals are volatile organic compounds. Organic in this context means derived from petro-chemicals, and are commonly referred to as VOCs. Synthetic rubber emits VOCs which can easily be identified by a plastic-like smell. VOC emissions are toxic, the effects of which are cumulative. In many cases the emitted toxins trigger allergic reactions. In some cases the reactions are extremely severe. Children are especially vulnerable to the effects of exposure. Latex derived naturally from the sap of rubber trees, is a much safer alternative.
Don't let anyone tell you futons are light-weight. They are extremely heavy and handle like a huge amount of dead weight. Safe turning of a futon requires two people.