Bug 1219855 switches to XUL alerts for the remainder of the session. That makes for an inconsistent experience across sessions if some notifications can use the system backend. For example, libnotify on Ubuntu doesn't support clicks or action buttons. If a notification doesn't use either, we'll use libnotify, and won't show "do not disturb" in Content preferences. Then, as soon as the backend fails, we'll switch to XUL, and DND will appear in preferences. We should think about making the fallback sticky, via a pref. We could expose a checkbox for this, but I don't think it's necessary.
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