A version for 136 is in the works:

https://github.com/thunderbird/import-export-tools-ng/issues/706

In the meantime, Help/Troubleshooting Info, Profile Folder, Open Folder, close TB, copy the mbox into Mail/Local Folders, restart TB, find the folder under Local Folders in the Folder Pane.

Thank you so much! I'll give that at try. And at least I now know what's going on.

I did it, but I don't see it in the Local Folders pane. When I downloaded TB a few days ago there were only two Local Folders: Trash and Outbox. That's still the case. They are both empty.

Forgive me if I'm being stupid. I'm new at this and don't know what different terms mean.

Wait, it looks like maybe it is working. There's still nothing under Local Folders, but I can see some gmail emails under All Mail, Sent Mail, and Inbox. Thank you! I will try to familiarize myself with this for a couple of days, and hopefully my problem is solved.

From your picture, the Archived-002 and Sent-003 files haven't completely downloaded, hence the .part extension. Once the download is complete, you should see files named Archived-002.mbox and Sent-003.mbox with non-zero sizes. Remove the .mbox extension so they have the same format as Trash and Unsent Messages, both of which are mbox files of Type File (remove it also from Inbox-004.mbox and fredericamg@gmail.com.mbox).

It still looks the same. I wonder if I should try the download again. I can let it go overnight. And it is sufficient to download only what is in the Takeout >Mail folder? "Multiple Formats" and "All Mail Data Included"? I don't run a business or anything--it's just personal email, but going back 20+ years.

The gmail All Mail folder contains messages from all other folders, and is usually the largest, so you can omit it if there's a choice to download just received, sent and archived mail.