Thunderbird default folder sort order

Before you visit any folder, you'll need to set these defaults. Any folder which has been viewed before setting them will kept its old settings.

You CAN right click on the headers, head right down the bottom and go "Apply Columns to..", but this reportedly does not do sort order. And also reportedly does not. Anyway, set the defaults so that any new folders get the correct settings, and then try out this UI method.

If that doesn't work, the nuclear option to get rid of the old settings, close thunderbird and delete any of the msf files from the profile directly.
find . -name '*.msf' -print0 | xargs -0 rm

NB This will delete your flags!

To set defaults:

Preferences > General > Config Editor

Search for:
mailnews.default

Resulting in:

Preference Name                      Status       Type        Value
mailnews.default_news_sort_order:    default      integer     x
mailnews.default_news_sort_type:     default      integer     y
mailnews.default_sort_order:         default      integer     x
mailnews.default_sort_type:          default      integer     y

x (see https://developer.mozilla.org/en/nsMsgViewSortOrder)

1 = Ascending
2 = Descending

y (see https://developer.mozilla.org/en/nsMsgViewSortType)

17 = None
18 = Date
19 = Subject
20 = Author
21 = ID (Order Received)
22 = Thread
23 = Priority
24 = Status
25 = Size
26 = Flagged
27 = Unread
28 = Recipient
29 = Location
30 = Label
31 = Junk Status
32 = Attachments
33 = Account
34 = Custom
35 = Received