Mail sorts from oldest to newest by default

This design choice escapes me and is highly annoying. Go go Preferences/Settings > Config Editor (the very bottom problem at the page)

It’s called Advanced Preferences tab. Look up mailnews.default_sort_order and change the value from 1 (ascending) to 2 (descending).

You can change the field to sort by mailnews.default_sort_type. By default it’s 18 (sort by date)

Thanks https://markohoven.com/2021/01/06/descending-sort-order-thunderbird/ for the clue.