If you click on the "Date" column header, you can toggle between oldest and newest at the top.

Thanks for the suggestion but I appear not to have made my problem clear. Up until now, for the several years I have used Thunderbird, under XP, and now Linux Mint, the e-mails in the Inbox have been listed in ascending date order but, when the Inbox was opened the most recent e-mail was highlighted by default - i.e. the vertical scroll bar thumb was at the bottom. Now the scroll bar thumb has suddenly moved to the top for a reason that completely eludes me. I want to retain the ascending date order but I want to return the situation where the most recent e-mail is shown when the Inbox opens.