If using up to date version 38.2.0 Previously, the 'Address Book' would always open on the default 'Personal Address Book' as it was the top one in the list. Then, after many requests by a group of people who wanted to be able to search all their address books in one go, a virtual address book called 'All Address Books' was created. This meant that it would always default to opening on the 'All Address Books' every time you opened the 'Address Book' even in the same thunderbird session. A bug request has been made asking for the ability to be able to set a default address book of choice and this is in development. At the same time, it was discovered that the 'Write' window Contacts Sidebar, would remember a selected address book providing Thunderbird had not been closed - only remembered in that session. So, they are also looking to set a default address for this as well, so that the Contacts sidebar does not auto default to 'All Address Books' each time thunderbird is closed and the session ended. The 'All Address Books' currently displays everything from all your address books including 'Collected Addresses'.