My guess is that you had Thunderbird running on the source computer when you did the copy. I suggest repeating the process but have Thunderbird closed. You're copying the entire profile so all entries would be copied as one.

I tried again and got a different result but not success.

Here's what I did: On PC: Used thunderbird troubleshooter to find default user and opened folder, Went up 3 levels, Closed Thunderbird application. Copied Thunderbird folder to USB drive. Got several messages about perf-xx.js folders having properties that couldn't be copied to new location (i.e., the USB drive); these folders had 0 bytes and I copied them anyway. Other perf-xx.js folders copied fine and had contents. (Note: xx was a number from 1 to 22.)

On Mac: Installed Thunderbird, Copied Thunderbird folder from USB drive Opened Thunderbird, was asked to create user, Created user, Opened Thunderbird again, got import wizard, selected import from previous Thunderbird, import ran fine, Some (more) of the folders are there but some have been scrambled or are missing. For example, all archive folders I created have been place under the archive folder where I had separate archive folders under other topical folders. All message filters are missing, too

Any other thoughts about what I did wrong?

Best regards, Richard

The fact that you indicate multiple copies of the prefs.js file tells me that your first priority 'oughta be to clean that profile. And I'm not the guru for that. There should be just one prefs.js file. maving many indicates that, over time, there have been problems. When I copy a profile in windows, I receive no such notices, so maybe Thunderbird was not fully shut down. After copying, you can use windows file explorer to view the copied profile to see the Mail folder and Imapfolder to see what is there. Local folders are in the Mail\Local Folders folder. The fact that the Mac prompted to create an email account is the first clue that it doesn't like the profile, possibly due to the multiple occurrences of prefs.js. BUT, all that aside, if you see missing folders in the Mac, you can use the OS's file management software (the equivalent to Windows' file explorer) to copy the missing folders in.