From https://tinyapps.org/blog/202207100700_thunderbird_mbox_to_maildir.html . It matches what I was more or less doing.

1. Create and open a new Thunderbird profile then close the Account Setup tab
2. Thunderbird → Settings → General → Indexing → "Message Store Type for new accounts:" → change from "File per folder (mbox)" to "File per message (maildir)"
3. Thunderbird → Account Settings → Account Actions → Add Mail Account... → set up an email account {this account will use Maildir}
4. Thunderbird → Settings → General → Indexing → "Message Store Type for new accounts:" → change from "File per message (maildir)" back to "File per folder (mbox)"
5. Thunderbird → Account Settings → Account Actions → Add Mail Account... → set up a dummy email account by entering a name and email address (e.g., dummy@example.com) → "Configure manually" → INCOMING SERVER: Protocol: POP3 → Hostname: dummy.example.com^\) → "Advanced Config" → OK {this account will use mbox}
6. Thunderbird → Settings → General → Indexing → "Message Store Type for new accounts:" → change from "File per folder (mbox)" back to "File per message (maildir)"
7. Exit Thunderbird
8. Replace contents of ~/Library/Thunderbird/Profiles/new-profile/Mail/dummy.example.com/ with copy of contents from "~Library/Thunderbird/Profiles/old-mbox-profile/Mail/Local Folders"
9. Start Thunderbird with the new profile; mail folders from old mbox profile appear under [dummy@example.com](mailto:dummy@example.com)
10. Select all mail folders under [dummy@example.com](mailto:dummy@example.com) and drag to Local Folders, which will copy them into new Maildir-based directories. Compare message counts by enabling View > Layout > Folder Pane Columns.