The bug for this https://bugzilla.mozilla.org/show_bug.cgi?id=571133 indicated it was fixed in https://bugzilla.mozilla.org/show_bug.cgi?id=505035 for V3.1.

So either there is a regression or there is an add-on affecting things for you.

Try safe mode (restart with add-ons disabled from the help menu

Thanks Matt. I had a look at the bugzilla correspondence. It was mostly about implementing a method of copying column settings from one folder to another, optionally including subfolders. This is still working correctly.

But in the discussion there was talk about where a new subfolder gets its column settings from. This pointed me in the direction of the source of my current problem.

Current behaviour: A new folder inherits its column settings NOT from its parent folder but from the Inbox folder.

For me, this is the same in most cases as I have all my folders, including Inbox folders, set to the same column settings. But I have multiple accounts, each separately IMAPed, plus a set of local folders. My local folders do not include an Inbox folder because I do not use POP, so new folders in my Local Folders collection get the default column settings instead.

Preferred behaviour: New folders should inherit their column settings from the parent folder.

I thought an inbox folder was a requirement on Local folders. If you have managed to delete it, create one.

The FolderFlags addon allows you to set the relevant folder flags to make a folder you create a "special folder" https://addons.mozilla.org/en-US/thunderbird/addon/folderflags/

matfeynisya said

Preferred behaviour: New folders should inherit their column settings from the parent folder.

File a Bug. It sounds like a more than reasonable arrangement to me.