So I’m still used to pressing WinKey+R (AKA Meta+R) to bring up a command prompt. I added the Search and Launch widget to my default panel in KDE Plasma, then right-clicked on it and selected “Search Settings…”. From there I could bind an appropriate keyboard shortcut.
Then I installed the Akonadi console:
root@tact:/home/jj5# apt-get install akonadiconsole
I opened it from K -> Applications -> Development -> Akonadi Management and Debugging Console.
Then in it I selected Agents -> Local Folders.
Then I changed: /home/jj5/.local/share/akonadi_maildir_resource_0 to my ‘important’ Thunderbird ImapMail directory.