Why shift-cmd-n fails as a custom shortcut for Firefox "New Private Window"?

I created the shiftcommandn shortcut for Firefox in Syst Prefs to match other browsers' behavior. It registers in the Firefox Menubar. But pressing the shortcut opens a new non-private window, and it loads there the first item in my "Top Sites". I imagine I could create another shortcut to override this behavior, but what is this behavior? I don't see anything highlighted in the Firefox Menubar when I hit the shortcut. Or might there be another solution (other than using a different shortcut to open new private windows)? Problem exists on both Mojave and High Sierra. Default Firefox shortcut (shiftcommandp) still opens a new private window.