My understanding is that the newer formula pressure treated wood no longer has the CCA components, that being said I don't know what they use now but it doesn't have near the toxicity that the old CCA ones did and the lifespan of the lumber shows it.
Until 2003, the preservative most commonly used in residential pressure-treated lumber was chromated copper arsenate (CCA), an extremely toxic chemical. During a fire, emissions of chrome and arsenic would make their way to the catalyst and especially arsenic, is a major catalyst poison.
Although the “post-2003” formulations no longer contain arsenic, your local home store or lumberyard is now selling lumber treated with less toxic alternatives... amine copper quat (ACQ) and copper azole (CA). Both of these contain copper, which is a catalyst masking agent. So the cats would last a little longer with the newer pressure treated, however eventually, the copper would mask the catalyst surface. The arsenic and copper are “inorganic”, they won’t volatize or burn like organic compounds will. The inorganic additives to make the pressure treated resistant to rot and vermin, do not dissipate over time. They don’t leach out much either.
Let me see if I get what you are asking: Are you concerned that the split firewood that comes into contact with the treated boards of the shed will absorb some of the compounds in that lumber? If that is it, I can't imagine there being a problem--as noted earlier, there isn't much leaching in pressure treated stuff. I would say that the lumber comprising the shed should be dry though. I bought some pressure treated stuff last fall, and it was soaking wet. The preservative would drip out any time I drove a screw or a nail into the wood. In a few months it was relatively dry.