Recently I have made the switch away from MS (OS and Outlook) 100% and and am on Ubuntu 22.04 (all latest patches and 5.19 kernel) and currently running latest Thunderbird beta 104.0b2. To be clear, I am NOT an email power user. I have three accounts and use them for nothing more than personal email and a single account for the calendar.
What is bugging me is that "Mark as Read" is only an on/off config. Meaning, either it is marked when seen or a delay. However, when I forward and email or open the message in a new window, it does not mark the email as read. In each case I need to manually mark it as read.
Again, I am on the latest beta and this may not be what prod actually does and/or could very well be something in the works already, and if so, mea culpa!