If you were REALLY concerned about rust and other corrosion caused by airborne stuff, you could always package your stored firearm(s) in vacuum bags. The systems (machines and bags) by
Foodsaver which we use for foods we are going to freeze would be perfect for most handguns, rifles, and shotguns. The bag material is available in long rolls and has enough cross-sectional area to go around most long guns.
Quite frankly, if I used one of the little "safes" for handguns, I wouldn't worry at all about moisture. For one thing, the whole thing would be in our air-conditioned home. For another, I haven't seen any of those things made in a way that makes them airtight.
One thing that can be used to prevent corrosion is the treated paper that I use for my woodworking chisels, planes, and other precision tools. If anybody's interested, I can try to find out where I got the huge box of treated paper (it was from a web site).