Hello,
Please update your thunderbird page here https://sabre.io/dav/clients/thunderbird/. Lightning is no longer available and its functionality now it's part of the core client.
I'll be happy to provide more details if needed, but the main inf is valuable here: https://support.mozilla.org/en-US/kb/creating-new-calendars#w_on-the-network-connect-to-your-online-calendars