I created those shiftcommandn Link for Firefox in Syst Prefs to customize the behavior of other browsers.
It is registered in the Firefox menu bar.
However, clicking on the link opens a new, non-private window, and loads the first item into Top Sites.
I could imagine that I could create another link to override this behavior. I do not see anything highlighted on the Firefox menu bar when I press the shortcut. Or could there be another solution (except another link to open new private windows)?
There is a problem with both Mojave and High Sierra.