Do you want to select (focus) a particular Inbox folder automatically on Thunderbird startup? You can do this easily with the add-on "Start with Inbox" https://services.addons.thunderbird.net/en-Gb/thunderbird/addon/start-with-inbox/

No "Start with Inbox does not do what I'm asking. A "Focused" or combined Inbox means that when selected you see all Inbox email from ALL accounts in ONE place. Have you not used Outlook? or AirMail? Both have combined email folders. I use Thunderbird on my Linux machines as the "Better" email clients are not available on Linux...

click view>folders and tick unified folders. then, click the 'inbox' and then rightclick and properties and then choose to confirm that you have selected the desired accounts repeat for sent, trash, etc.

English is not my mother language. Concerning Thunderbird I have never read the term "focused inbox". As an exemple: in my understanding a focused folder in the folder pane is a selected folder which at the same time is in the foreground, but you may have a completely different interpretation. I have never used Oulook nor Gmail nor Airmail (but from Netscape to Mozilla) but "Combined Inbox" tells me something. Thunderbird uses two expressions: • when you have POP accounts you can combine them in the Global Inbox generally in the Local Folders • many users choose "Unified Folders" which allows you to combine POP and IMAP accounts