All right..not a terribly well-written original post...but let me see if I can shed a little light based on what I think she's asking, with an example.
Most states do not "absolutely prohibit possession of a firearm." But my state, Massachusetts, does require EVERYONE to obtain either an FID (Firearms ID card) or LTC (License to Carry). Which one you need depends on what kind of firearms you want to own. If you don't have an FID or LTC, you are prohibited from purchasing or possessing a firearm of any kind.
But let's say, for the sake of argument, our imaginary friend Joe is a Massachusetts resident who inherited Grandpa's old Colt 1911 that he smuggled home from WWII. And let's say Joe is a "hardcore constitutionalist" who carries that pistol everywhere he goes, regardless of state laws. And to keep it simple, let's say Joe has a clean criminal record, no felony convictions. Joe does not have an FID or LTC of any kind.
If he is caught in Massachusetts, they will prosecute the hell out of him, because he is in violation of a number of laws.
However, if he is visiting Vermont, he cannot be prosecuted for anything. Vermont does not require a license for possession of a firearm, nor for concealed or open carry. Vermont cannot prosecute Joe under Mass laws; only under Vermont laws. Each state's laws apply only within the borders of that state.
If he is visiting Montana, he can be prosecuted only under Montana law.