The Sort menu contains three items: By article number, By thread and a checkbox to specify the list order (ascending/descending). By default, article headers are listed in descending order by article number. If you select Sort->By thread, the articles are grouped by threads in descending order, e.g.,

Note that "orphaned" articles (those whose parent article does not appear in the list) may not necessarily get grouped with articles having the same subject line.
To reorder the list by article number, selecting Sort->By article number:

or, to list in ascending order, select the Ascending checkbox:
