In private browsing mode and with LWT's applied, window modal dialogs anchor to the very top of the window. We should probably anchor to the bottom of the titlebar instead, which (when tabs are in titlebar) is stretched down behind the tabstrip. Our native widget code tells OS X where to anchor the window modal dialogs by setting -moz-appearance to -moz-window-titlebar on the element to be anchored to. That -moz-appearance is removed in private browsing mode or when LWT's are applied. We might consider putting that -moz-appearance on a pseudoelement on the titlebar when in those modes, with its visibility set to hidden.

I'm not sure but I don't think this is an Australis regression (at least for one of the two modes). Note that it wasn't in the dependency tree anyways.