I do not recall a time when there was a setting in the user interface to set the location of individual folders (going back to version 2 anyway) There always has been and still is a location for account storage in the user interface. SO perhaps you could explain this in more detail. Did you perhaps set the entire local folders to the D: drive, or move you profile to the D: drive The later would certainly free a lot of space on an small SSD used for the system volume.

It may have been created at least ten years ago so I don't remember how I did. Must have been legitimate though. Here is how part of my mail folders look like (in Danish). The names in bold were created by Thundermail. Drafts, Templates, Sent, Archive, Spam, Waste, Archive, D:\Mail, imap, Dubious, Local folders

Can you right click that folder and the one above it (archives) and select properties and post what it shows in the Location text box for each.

I notice that the folder appears in italics as does the IMAP folder below it, something which is normally reserved for a folder that is read only in an IMAP mail account.

The context in not clear in this image.

The fact that Local Folders appears after the folder names indicates the folders are sub folders of some other account. What account? And is it an IMAP account?

Here are the properties of the two folders. D:\mail contains mails from the same account. The idea was to free the space on the server, but I am not sure this has worked. I tried to disconnect the D-drive (an external drive), go to the imap server and select a mail from D:\mail and it did retrieve it. So it must be on the server too. I guess I might as well create a normal folder "Old mails" in Thunderbird, move the mails from D:\mail into Old mails and delete D:\mail - since the idea did not work and since it seems partly to violate how Thunderbird should work. Maybe I should also change the profile so that the whole Thunderbird mail were on the D-drive

Maybe I should also change the profile so that the whole Thunderbird mail were on the D-drive

That would probably be a good idea. Note that any folder you create in local folders and move mail to will be stored locally on your machine only in whatever location you set for the profile. So local folders does what you originally set out to do, relieves the storage requirements on the mail server.

You can even set a local path for the local folders account that is separate from the main profile folder but having the whole profile together in a location is preferable from a support perspective as nothing is accidentally get left behind when you move a profile, say to a new machine.