But I'm not doing pacman -Sy package. That is not what I am talking about. I am only talking about pacman -S package, which is not updating the system partially. IF the package depends on something else to update, then the system would need to be updated. But that is not what I was asking, because I only talk about the package with -S package. I just chose firefox as an example, it could have been any other package.
To make it clear, when I say -S firefox, then I mean really that without updating a dependency like libssl. The idea is to install only new packages without updating anything on the system. I guess as you say it depends on the dependencies of the package, if this is feasible.