Here is some information that I haven't tried myself, not being a Mac user: To sync an iCloud calendar with TB/Lightning: https://frightanic.com/apple-mac/thunderbird-icloud-calendar-sync/ Add an iCloud calendar using TB/Lightning and these two add-ons: https://addons.thunderbird.net/en-US/thunderbird/addon/tbsync/ https://addons.thunderbird.net/en-US/thunderbird/addon/dav-4-tbsync/ The second method with add-ons may be as simple as installing the add-ons and entering your iCloud credentials.