Thunderbird does not have a function to import mbox files, so please be specific on the steps you are taking. Thank you.

Import from Apple Mail never really worked except in very old Thunderbird versions. This is still true for actual Thunderbird versions and my Mail.app version 16.0 running on macOS Monterey.

But there is a workaround: In Mail.app select a folder, e.g. an Inbox folder, R-click and choose "Export Mailbox". This will open a Finder window in which you'll choose the destination folder, e.g the Desktop.. The result of an Inbox export will be a folder INBOX.mbox (or Sent.mbox for a Sent export) on your Desktop. If you open INBOX.mbox with a double-click you'll see two files (see image): a file named mbox which is a text file containing the Inbox messages (similar to Thunderbird mbox files), and a table_of_contents file which is somewhat similar to an index file.

You can import those mbox files in Thunderbird with the add-on ImportExportTools NG or manually. I prefer the manual method: open the Thunderbird profile folder and quit TB, go to .../Mail/Local Folders/ and drag & drop the mbox file from your Desktop in Local Folders. When you restart TB you'll see a folder "mbox" in your Local Folders which you can rename as you like. To do this in a more efficient manner: you export several (or even all) of your mailboxes from Mail.app one after the other and rename the mbox files at once as you like, for instance Inbox-1, Inbox-2, ... Sent-1, Sent-2, Drafts-1, :.... etc. Then you open the profile folder via Users/username/ Library/Profiles/xxxxxxxx.default-relase/Mail/Local Folders/, drag & drop all those exported and renamed mbox files in Local Folders and restart TB where you can reorganise your imported Mail.app mailboxes

Accessing the Hidden User Library (macOS) https://support.native-instruments.com/hc/en-us/articles/360020012418-Accessing-the-Hidden-User-Library-macOS

Export Mail.app mailboxes https://support.apple.com/bg-bg/guide/mail/mlhlp1030/mac

I beg to differ, david, but Thunderbird does have an import from Apple Mail radio button as evidenced by the screenshot.

Thanks, Mapenzi, for your well-thought-out answer. I exited Thunderbird and tried your manual method. I’m attaching a screenshot of the Local Folders and its contents with no e-mails yet, right where you said it would be. However, when dropping mbox files into that folder, nothing shows up when I relaunch Thunderbird.

I have Thunderbird on an older PC with a different e-mail address and love it. That’s why I thought I would try it on my Mac because Outlook and Apple Mail have failed me. Mozilla has no tech support phone number nor e-mail.

You didn't follow my instructions or you misunderstood them. Your last screen shot shows the contents of Local Folders in the Thunderbird profile folder. It contains an Inbox file of 35.7 MB size. So if you restart TB now you should see an Inbox folder in the Local Folders in the folder pane The Local Folders in your screen shot is the repertory in which you should drop the renamed mbox files created by your export from Apple Mail. Look at my first image which shows my Local Folders in the profile where I have dropped four exported and renamed mbox files from Apple Mail: Inbox1, Inbox2, Sent1 and Sent2. Is this clear for you until this point?

Now I restart Thunderbird and look at my Local Folders in the folder pane (second image). There are four folders with the names Inbox1, Inbox2, Sent1 and Sent2 and their contents.

Hello Mapenzi, I obviously misunderstood your instructions. The small .png files that I didn’t notice from your first reply helped me understand which folder to drop the renamed mboxes into and now I’m up and running on Thunderbird with most of the e-mails. A few did not come over in the transition from Outlook to Apple Mail to Thunderbird, but that’s OK. Thank you!