Users should not have to wait while shutting down Firefox. This means that we want - all windows to be hidden immediately, (this overlaps with the 'close windows immediately' goal of bug 1363767). - the profile lock to be gone as soon as possible. To a lesser extent, we want the process to exit soon, but that's probably not user-visible, unless the user is frustrated enough by one of the 2 above points taking too much time.
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