When using Thunderbird (versions 102 and higher) you do not need any special addons. Thunderbird core can synchronize using the "vcard" and "vcal" standards.
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Click the "Address Book" icon in the top of your window

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Enter your username that you use to login via https://share.mayfirst.org/ (note, the user must be configured with Nextcloud access in the control panel).
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For location, enter: https://share.mayfirst.org/.
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Choose the contacts you would like to synchronize.
- Click the calendar button in Thunderbird.
- Click the "New Calendar..." button in the bottom left side of the screen.
- Choose "On the network".
- Enter your username that you use to login via https://share.mayfirst.org/ (note, the user must be configured with Nextcloud access in the control panel).
- For location, enter: https://share.mayfirst.org/.
- Do not check "This location doesn't require credentials".
- Do check "Offline Support".
- Choose the calendars to synchronize.
- Do not check "Prefer client-side email scheduling"