When adding a Thundermail account to Thunderbird or other email clients, that client will prompt you to add your account’s contacts / address book data, which is handled by an Internet protocol called CardDAV (vCard Extensions to Distributed Authoring and Versioning).
Through CardDAV, you will be able to connect as many clients as you wish to sync with all of your personal and shared Thundermail contacts.
If your client does not automatically import your contacts information when you add your Thundermail account, you can enter your CardDAV URL manually into its settings.
That URL is formatted like this:
https://mail.thundermail.com/dav/card/<userid>%40thundermail.com/default/
For example, if your Thundermail address is example@thundermail.com, your CardDAV URL is:
https://mail.thundermail.com/dav/card/example%40thundermail.com/default/