Outstanding Rated 5 out of 5 stars
A restart of Thunderbird makes the fault I mentioned before go away. So it works without flaws if a new subfolder is created, then TB is restarted before trying to use QuickFilters to drag message into the new subfolder of a QuickFolder tab.
I know it's made to drag into the folders pane of TB instead of the tabs of QuickFolders, but it works both ways.
Thanks for great add-on.