on the command line, you can use dpkg --listfiles packagename. For instance, dpkg --listfiles firefox. If you want to see what files a package contains without installing it, then you can install apt-file and use that.

But you really shouldn't mess with it. There is usually no reason to manually interfere with the contents of a package. All configuration files for normal applications are placed in the users home directory. You don't have savegames in C:ProgramfilesAppnamesavegames, for instance. They would be placed in /home/username/.local/share/appname/savegames. That way, if you move your home directory to another machine, it keeps all configurations and user data.