printscreen added

I think you can just drag and drop it.

But you need to understand that a Mailing List is just a subset or collection of Contacts from its parent address book. So a good starting point would be to move or copy the List's member Contacts to the new address book beforehand.

If you don't do this and just move the Mailing List, it will automatically create new minimal Contact entries in the new address book to match those brought in within the List. This will lead to duplication of those Contacts in your Address Book, most noticeably when viewed as "All Address Books".

Note that quite often you will move something in the Address Book and it will appear not to move. Try closing and reopening Thunderbird before doing anything desperate.  ;-)

BTW, I would refer to NHAV as an "address book" rather than a folder. People often use "folder" to mean a Mailing List, because a Mailing List is presented as if it were a subfolder of some other address book. We have to patiently decide what people mean when they say list, folder, group etc.

Even "address book" is unsatisfactory as a name because in itself an "address book" is a subdivision of the Address Book. But given that the default primary subdivision is called "Personal Address Book" we're kind of stuck with the internal subsections being referred to as address books.