Is TB being run on a Mobile, Tablet or Computer?

Computer

Although Mobile Apps often include the previous correspondence with the email, I wasn't aware that TB on a Computer did this - and can't find a setting.

However, did you mean that several emails in a list of correspondence are listed line by line rather than as one? If so, have you accidentally clicked on the Thread Toggle (attachment)?

Thank you for responding. Much appreciated. Your reference to a "Thread toggle" provided a clue. Initially I did not know what you were referring to as I had never seen a Thread toggle. Eventually I found the setting for it. After a lot of experimenting I finally wound up with a display that groups together emails with a lot of responses to the same subject at the top of the email list under the Subject heading with the toggle symbol and a carrot to expand/collapse the group. Annoyingly the groups are not displayed chronologically and the expanded groups are displayed in ascending order. New emails are entered before the groups putting the groups between previous emails with no thread and new emails. Really poor and confusing display. This display is not what I hade before but at least now with the responses grouped I do not have all the individual responses displayed along with all the other emails.

My original display had only the original email with the return symbol (left arrow). My reply was not displayed and there was no "group" sign. All emails were displayed in chronological order. Have no idea why the sudden change. I had not made any changes to Thunderbird's settings. Was there some recent update that I am not aware of?

Bye the way on one of my experiments with different settings I got a display that listed the number of emails received during a specified time period e.g., "n" email 2 days ago, "n" emails 7 days ago, etc.. No idea how what settings were used and have not been able to recreate it.