We should use hiddenPrivateWindow now.

Actually, this doesn't matter, since the preload page is not privacy sensitive.

I do wonder if there should be a new utility function along the lines of getHidden[DOM]Window(window) and we deprecate the appService (IIRC) one. ISTM there is no real risk of a private window using the hidden private window even when things aren't privacy sensitive, but there is a risk in people needing to understand the privacy implications before choosing which one to use. IOW, it seems we should move to a scheme where developers need not make this decision and they just get the correct one automagically...

That's not a bad idea. What we need there is a) a new bug, b) a good place to put that function in, and c) someone who wants to write the patch and update the callers. Things to remember: not every place in our code base where we use hidden windows has a notion of the current window. In fact, many of those callers do use the hidden window just to get access to a window which they can do useful things with, such as creating DOM elements through hiddenDOMWindow.document.createElement.