What does "folder for an email account" mean?

This sounds as if you want to remove an account. Go to the Account Settings page, select the unwanted account, go down to the bottom of the left-hand narrow pane listing your accounts and click on the button. It may or may not be labelled "Account Actions" but it's currently (bizarrely IMHO) configured to tell you what you last did with it.

I think it's quite right and proper that deleting an account, and potentially all the messages stored inside it should require more than a single click. We have enough trouble with users accidentally deleting things as it is.

Thank you, but, no, I want to delete a folder within an offline account, not the whole account. I store what looks like spam in monthly folders in case someone I know asks why I didn't respond to an email I thought was spam. I don't think I've been asked that in years, but I generally keep them for a month or two, which means every month I want to delete the older month of probable spam. It's no big deal if I have to empty it first but then I should be able to delete the newly-empty folder. Any idea how?

Well I have just deleted an empty folder and anther folder which was not empty. Both just quietly vanished and stayed gone.

I right-clicked and used "Delete". But on yet another folder, "File|Delete Folder" worked too.

Who is your email provider? ISTR you mentioned Yahoo in another thread.

I have also tried rename via both routes (File and right-click) and again, no problem, nothing greyed out. And this was on a Yahoo account.

Following on from a point raised in another thread, are you online when trying to do this?

Yahoo.

Offline (when deleting fails). I don't even want to test deletion in Thunderbird while online, because that would risk deleting from Yahoo at the same time, and if it doesn't risk it then being online shouldn't be relevant.

Is it possible that since I had already deleted a folder in Yahoo then Thunderbird is not letting me delete the formerly-matching folder, because they're not sync'd? I don't want to synchronize them, because I want to delete emails from Yahoo while keeping them locally through Thunderbird. I plan to move the local copies to CD when I know what to move.

I right-clicked (context-clicked) the selected folder after your post and Delete and Rename were still dimmed. In the top-of-window File menu, Rename was dimmed but Delete Folder was available and selecting it got a confirmation, but confirming, waiting, quitting, and restarting Thunderbird showed the folder still present; the same happened with the Thunderbird (hamburger) menu. I've cold-rebooted at various recent times.

Is this POP or IMAP?

If it's IMAP then it is entirely intentional that the server and client are synchronized and what you do on one is mirrored on the other. So no amount of this yoyo-ing between offline and online will ever make any difference because at some point in the future the two systems will see each other and set about becoming consistent with one another. What you have deleted on the server will be deleted in the client.

The answer to this in a situation where you want to clear the server yet make a local copy is to store the messages in a place where synchronising doesn't occur. In Thunderbird this is best done by moving messages to the Local Folders account

IMAP. Thank you for that advice, given before I would lose my local email copies. I'll try to figure out how to implement that advice, especially if I set up Thunderbird to work with a second email account and I want to keep the local email copies separate by online email account.