mails should be sorted by 'date-descending'
This is a UX bug.
By default Thunderbird sorts all folders (Inbox...) by 'date-ascending'. This results in one big problem: new mails are always at the bottom of the Inbox = when having ~2,000 mails you have to scroll down pretty long to see your new mails.
Thunderbird is the only mail client I know that does sort mails by 'date-ascending' and I think this is a big negative point because Thunderbird acts in a way the user is not expecting.
I support an elderly man using his computer and having new mails at the bottom was very confusing form him. Most average people I know never configurate the programs they're using and therefore I don't think that they would have found 'View > Sort by > Descending'
Change the default view from 'date-ascending' to 'date-descending'