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Lightning has disappeared. It appears as an add on but it's not working.

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Lightning calendar and tasks column has disappeared. It appears as an add on. But the column does not show up. Also does not show up in preferences anymore. I'm using a mac, el capitan. Thunderbird version 38.7.2. Lightning version 4.0.7.2.

Lightning calendar and tasks column has disappeared. It appears as an add on. But the column does not show up. Also does not show up in preferences anymore. I'm using a mac, el capitan. Thunderbird version 38.7.2. Lightning version 4.0.7.2.

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Have you tried removing Lightning altogether and reinstalling it? (Not reinstalling Thunderbird, just the add-on.) I suggest remove > shut down and reboot computer > reinstall, in order to refresh the settings.