Yes, Serta memory foam toppers are toxic, but then so are the foam toppers made by any other company. Frankly, any petrochemical products (foam and memory foam included) are poisonous, and you would not want to eat it or burn it and breathe in the fumes. That’s why you should not just toss your old memory foam pad into the burnable trash. In some municipalities, it may be necessary to send your memory foam mattress pad for specialized disposal.
That said, if your main concern is about VOC fumes released by Serta toppers, set your mind at ease. After you air out your memory foam bed topper for two weeks in an open space, any VOCs that were built up during storage will be cleared away. This is true for any brand, even Tempurpedic mattress pads. After two weeks of air-ing, any chemical you smell is likely to be from the flame-retardant treatment mandated by government.
If the smell of your memory foam mattress topper is still very strong after two weeks, just return it to the shop where you bought it, or get it exchanged. If you bought it locally, you can point out the strong difference in smells from the set in the showroom.