The importance of a restful night's sleep and the investment in a mattress are not small matters. When your mattress warranty doesn't provide the option of returning a too-firm foam mattress, you'll have to look at the alternative of keeping the mattress but modifying it.
The Base Support
Change the base support for your foam mattress to something less rigid. For example, if your base is a traditional box-spring foundation with a large piece of plywood between it and the foam mattress, try a piece of plywood that is not so dense or is more flexible, such as particle board.
Memory foam is not supposed to be firm but to provide an even weight distribution and elimination of pressure points for greater sleep comfort with adequate support where needed. While it is true that foam-mattress bases should be totally flat, the rigidity of that flat surface base may the reason it feels too firm rather than the mattress, itself.
Place a layer of softness, whether foam, feather, wool or cotton, between you and your too-firm mattress. Pillow-top mattresses and toppers are popular for a reason--they make practically any kind of bed more comfortable. A luxuriously soft mattress topper may be the most affordable, practical and satisfying solution. ??
A foam topper on a foam mattress may sound strange, but may be a good way to go. The thicker the topper, the more softness you'll enjoy. However, density is also a factor and foam toppers come in different ILD (Indentation Load Deflection) ratings, from 18-22 (soft) to 34-38 (firm). ??
Feather or down mattress toppers will give you softness without support--a particularly good option for a firm mattress that needs cushioning for comfort.
Wool toppers work the same as feather, providing a layer of comfort that is naturally hypoallergenic in a variety of blends, including lamb's wool, alpaca. Wool is generally more expensive but cotton is a popular hypoallergenic topper and an easily cared for down alternative.
Organic fill and outer coverings will be free of chemicals, dyes and bleaches, but will compress over time, despite regular fluffing.
A thick and plush mattress topper may result in the need for new sheets, as well. If your fitted sheets are already snug, the topper may require deep-pocket fitted sheets.