That there are two different shortcuts for re-opening closed tabs and closed windows has two problems:
1. You have to remember if the closed page was in a closed tab or a closed window. To have only one keyboard-shortcut for both, would prevent this.
2. Because ctrl+shift+n is used for re-opening closed windows, it cannot open a private window like in most other browsers. That makes switching to Firefox unneccesarily hard.
The better way:
using the same shortcuts as other browsers. ctrl+shift+t for re-opening both, closed tabs and closed windows, in reverse order. And ctrl+shift+n for opening a new private window. That would solve both problems described above.