[ken]

For Firefox on OS X, at least, it seems to be reporting the resolution of the display which the window is mostly on.

I think this corresponds to the display which holds the backing store for the window, but I'm not sure -- if you drag a window between a HiDPI display and a normal display, for example, it's rendered as a HiDPI window if it's mostly on the HiDPI display, and vice versa.

I agree that I don't know if it's possible to do the "right" thing, or even exactly what the "right" thing would be.