Close TB, open the profile folder, and delete xulstore.json and folderTree.json. When you restart TB, do the folders appear?

No, it looks the same after deleting those two files.

Moving a profile from 68 to 78 is seldom straightforward, which is why auto-updates have not yet been deployed. You would be better off installing the exact same version of 68 on both computers, then proceeding as before: deleting the contents of the profile folder on the new computer, then copying in the contents of the profile from the old computer. All versions can be downloaded here:

https://archive.mozilla.org/pub/thunderbird/releases/

If you wish to stay with 78, I recommend you create a new profile and copy mail and address books from the old profile, manually.

Actually, it should make no difference whether a user installs 78 on a system that was running 68, or the user copies a profile to a system that has already run 78 on it AS LONG AS the entire profile is copied in one shot.

Looking at the screen shot, I would think that some antivirus is interfering with the login to imap servers. In fact I just had this happen a couple days ago on a Mac.

If I try manually copying the mail and address books from the old profile to the new profile, which files or directories do I copy? Just the Mail directory and the abook*.mab?

Windows Defender is on by default and I suppose McAfee is running too given what I see in Task Manager. There haven't been any alerts or pop-ups hinting at either of them blocking anything.

So, my plan is to uninstall v78.2.1 from the new computer, install v68.11.0 which is the same version that's on the old computer, delete the contents of the newly created profile folder on the new computer, and copying the contents of the old profile folder into the new one. I'll try this plan in the morning unless someone thinks there's hope for v78.2.1.

You can copy abook.mab and history.mab from one profile to another, but the other address books should be exported in LDIF format (Tools/Export in Address Book) and imported in the new computer (Tools/Import...). Copy the mbox files in the account subfolders of Mail and also Mail\Local Folders - the large files with no extension, named after folders - into Mail\Local Folders in the new profile.

You should only have one of Defender or McAfee running, and for either one the profile folder should be added as an exclusion.

BTW, if the Thunderbird migration process isn't working for you, it would be excellent and wise to report it in the bug system so it can be solved. Because at present I don't believe we have any confirmed reports of issues.