Does Thunderbird have a way to manually expunge a mailbox?

I admit that I may not understand what your intent is... does not one the settings found in <Account Settings | (account) | Server Settings | Server Settings | "When I delete a message..."> and/or <Account Settings | (account) | Server Settings | Message Storage | "Clean up ("Expunge") Inbox on Exit"> do what you want?

No.

• Using "When I delete a message -> Move it to this folder" doesn't delete the messages, it just moves them to a different folder ... where they might never be deleted.
• Using "When I delete a message -> Remove it immediately" prevents me from being able to un-delete messages that may have been deleted by mistake.
• Using "Clean up (expunge) Inbox on Exit" has two problems: (1) it only works on "Inbox" folders - or if it works on all folders, the description doesn't say that ... and (2) it only expunges when you exit Thunderbird.

What I'm looking for is ...

• A way to manually expunge messages ...
• from any folder, even if that folder is not an "Inbox" ...
• without having to quit and restart Thunderbird.

Again, every other MUA I've used for the past 20+ years includes a function like this. For example, macOS Mail.app calls this function "Erase Deleted Items".

Also, there are times when I may need to mark messages as deleted but NOT expunge them when quitting Thunderbird, especially if I'm only quitting Thunderbird to do an OS upgrade and I know I'll be picking up where I left off with Thunderbird afterward.

Delete files in any folder > right-click the folder > Compact folder to regain space. But you also have to delete the files from Trash to expunge them.

If it's an IMAP Trash folder, most or all providers will permanently delete messages after some period of time.

You can set you own retention policy per account (see <account settings | synchronization & disk space "to recover space...") or per folder (right-click on any folder, choose &lt;properties retention policy&gt;. < p></account>

The Compact option only applies to MBOX format folders.

@Lin

• I'm not talking about a "Trash" or "Deleted Items" folder - this is primarily about the "Inbox" folders in multiple accounts, although I do sometimes delete messages from (and want to expunge) other folders as well. The command I'm asking about needs to be available for every folder, although if it's labeled something like "Empty trash" for a "Trash" folder, that's fine as well.
• I can say with 100% certainty that there is nothing running on the server that deletes messages with the "deleted" flag, because I own the server, and am very intimately familiar with the software it's running (see qmail.jms1.net if you're curious). I made a decision 15+ years ago that clients (meaning "people who were paying me to host their mail", as opposed to "software accessing an IMAP mailbox") would be responsible for expunging their own folders, since I didn't want to accidentally delete messages from clients' mailboxes. (And even if there were a "janitor" process, it would have nothing to do with the question I was asking. I suspect you were reading more into my question than what I had actually asked.)
• Now that I'm home, I've been able to verify that the accounts in question are all using Maildir for local storage. The "Compact Folder" option is present on the right-click menus again, so I'm curious where your statement that "The Compact option only applies to MBOX format folders" came from.

As I mentioned in my response (above), I had originally asked the question because the menu I was seeing on the screen when I right-clicked the folder, (1) didn't have the "Compact Folder" option, and (2) was being drawn using the macOS native style, rather than Mozilla's "flat" style. Rebooting the machine seems to have fixed whatever the problem was, the "Compact Folder" menu item is present again. My apologies for not investigating in more detail before asking the question.

Thanks everybody for your help.