Currently (v120.8), when one right-clicks on a sender's eMail address in the header and selects "Add to Addressbook", the contact is created in the Personal Addressbook which, in my case, contains nothing because I use Google Contacts so my default addressbook (and there's a checkbox for the default setting) is that addressbook.
So the added contact ends up in the wrong addressbook and I have to remember to manually drag that contact record from the "Personal" to the Google Contacts addressbook.
I imagine one might consider this a bug but, if it's not, how about changing TBird's behavior to respect the "default" addressbook?