You should uninstall 73, restart the computer, then install 60 or 68, restart the computer. This won't affect your data in the profile folder. Then, Winkey+R, type thunderbird.exe -p --allow-downgrade when TB is closed. This will show the Profile Manager, where you can choose the profile and run TB 60 or 68 in that profile.

When I went to choose default for ver. 68 in Profile Manager, I got a dialog box saying that I am trying to open an earlier version of thunderbird.

sfhowes, since default results in a pristine Thunderbird, but enables me to get to Profiles, and I have ver 68.4 on my laptop with same data, would transferring Thunderbird data from my laptop to my desktop be a possible solution?

Yes, you could copy the Thunderbird folder from the laptop to the desktop:

Copy the Thunderbird folder on the source computer:

C:\Users\sourceusername\AppData\Roaming\Thunderbird

Delete the existing Thunderbird folder on the target:

C:\Users\targetusername\AppData\Roaming\Thunderbird

Copy the source Thunderbird folder into Roaming on the target:

C:\Users\targetusername\AppData\Roaming

Try to have the same version of TB installed on source and target, or at least the target version newer than the source version. Keep TB closed during the copy and paste. Use an NTFS-format drive for the copy if your profile has files larger than 4GB.

Note that even if you copy from one computer to another with identical TB versions, you may still get an error about 'older versions'. In that case, I've found that it's sufficient to click Exit on the error, then open Profile Manager from Winkey+R, thunderbird.exe -p and select the profile.

4freebird said

When I went to choose default for ver. 68 in Profile Manager, I got a dialog box saying that I am trying to open an earlier version of thunderbird.

then for some reason the

 --allow-downgrade

is not being recognized, but I've never heard of this problem.

Try --allow-downgrade before the -p ?

sfhowes, you were correct that in copying to target computer there was a dialog box that I was trying to start an earlier version. Using your sentence via winkey+R did not change to ver 68.

Then tried Wayne Mery's thunderbird.exe --allow-downgrade -p did let me open version 68. But it was virtually blank. No inbox, sent files, trash, or anything on the left side. When I was inputting the linked email, at POP it was not accepted, but at IMAP the hamster is still spinning.

Will think about situation as I go asleep now.

Don't know why, other than possibly computer restart after the winkey+R, but this morning computer started in version 68.4.