For the simplest setup, open Settings → Security → App & device access, name the credential for this copy of Thunderbird, choose Email & contacts, and create the password. Copy it when LightDrive displays it; the value is shown only once.
When you add the LightDrive email account with your full address, Thunderbird can discover the related CardDAV address book. Use the same Email & contacts password when prompted for email or contacts.
If Thunderbird does not offer the address book automatically, open its Address Book, choose to add a CardDAV address book, and enter:
https://dav.lightdrive.one/USERNAME/contacts/as the location;- your full
[email protected]address as the username; and - the same Email & contacts password created for Thunderbird.
You may instead create separate Email-only and Contacts-only passwords, but Thunderbird will then need the correct password for each service. Your main Vault password and Vault app passwords do not authenticate to email or CardDAV. If Thunderbird has cached an old credential, remove or replace the saved password when it prompts again.
After discovery, name the address book LightDrive Contacts. Create one test contact in Thunderbird, synchronize, and confirm that it appears in the Vault Contacts app.
Thunderbird can also use the address book for email recipient completion. That does not require copying contacts into the LightDrive mailbox.