Open the Address Book and simply go to the File menu > New > CardDAV Address Book, enter your Google account's username (full Gmail address) and click Continue. The rest should be straightforward.

Many thanks easy when you know how

I tried this suggestion because I can't get the Thunderbird address book to sync with GMX contacts. The result was my GMX contacts were imported into a new address book in TB which isn't what I wanted. Is it not possible to have changes in my TB address book reflected in my GMX contacts?

I tried this suggestion because I can't get the Thunderbird address book to sync with GMX contacts. The result was my GMX contacts were imported into a new address book in TB which isn't what I wanted. Is it not possible to have changes in my TB address book reflected in my GMX contacts?

The default address books are local only, they can't sync with remote address books. After you've added your remote address books (GMX contacts), you can copy contacts from the default address books to the GMX one then delete them from the default books, then going forward, you should be using the GMX one for saving new contacts. You can do so by changing the Addressing settings in Preferences accordingly.