Thunderbird 91 comes a little over a year after the Thunderbird 78 series, and brings a revamped account setup wizard with support for setting up signatures, encryption and CalDAV calendars, a new user interface for adding attachments, the ability to change the order of accounts in the UI, and the ability to redirect emails.
This new Thunderbird release also allows you to encrypt your emails to BCC recipients, though these will be exposed in the list of keys. New keyboard shortcuts are now available to access the To, CC, and BCC fields of the compose window, and Thunderbird now allows the showing of empty CC and BCC rows in the compose window.