The issue with Thunderbird 68 is it contains some "features" that bled over from the Firefox core developers. This means that is has code that want to use a new profile all the time for some really odd reasons, one of which is when the "bitness" of the Thunderbird used changes. 32bit to 64 bit.

At this point my suggestion is as follows, but really I am coming to terms with the changes as well. So I might get a little lost here.

Start Thunderbird. Use the troubleshooting entry in the help menu to open the [profile folder Thunderbird is now using post install. Close Thunderbird. Check windows that the process is actually not running (alt+Ctrl+Delete) and select task manager from the menu. locate your profile on the USB drive. Open it so you can see the prefs.js file that the profile folder contains. Copy all files and folders from that location into the profile folder you just opened from Troubleshooting information. As you are replacing one set of Thunderbird files in a profile with others there should be multiple requests to replace files which you should acquiesce to. Failure to see a prompt will mean you are copying files at the wrong "level" and not replacing one profile with another. Start Thunderbird.

YES! I discovered the same trick this morning and then found the alert for your post when I opened T'Bird. I've wasted days trying to find my mistake. THANKS!

But there's still another bug. T'bird is installed on C: (SSD) in my new PC, but I want to move the profile to the HDD because it's quite large with all my email accounts. So I copied the now edited default profile onto my HDD and used the profile manager to select it and set as default.

All looked good until I launched T'bird and opened the profile folder. The path is still to the (edited) profile on C: so it didn't accept the new path.

They need to fix this! And there has to be an easier way to import profiles, just by copying it into the right folder. This profile manager is STUPID!

After you copied the profile to the HDD, did you use Profile Manager to create a new profile and browse to the location on the HDD?

http://kb.mozillazine.org/Moving_your_profile_folder_-_Thunderbird#Use_the_Profile_Manager_to_move_your_profile

Yes, that's exactly what I did. I copied the default profile I "modified" onto my HDD. That profile works correctly now on SSD.

Ran T'bird.exe -p and created a new profile and browsed to the new location. It seemed to work but when I restarted T'bird and opened the profile folder, the path is still to the profile on C:

Did you follow step 10. in the kb article and browse to the account folder for each account in Server Settings, in the new profile's location?

You might want to use the About:profiles link in troubleshooting information on the help menu. It will probably get to be the profile manager replacement.

I have some notes on using it for lost profiles here https://thunderbirdtweaks.blogspot.com/2019/09/i-lost-my-profilemail-on-update-to.html

I fixed it, but this is so weird ...???

Summary:

• To import profile from old PC, I copied/pasted its contents into the profile T'bird created on new PC in C: ....\roaming\thunderbird\profiles\. Couldn't just select the old profile with its original name copied into same folder on C:.

• Copied that edited profile (using same name) to D: but couldn't get T'bird to use it. Kept using the one with same name on C:.

• I renamed edited profile on D: back to the name it had on old PC. And now T'bird accepts it and shows profile path to be D:\appsdata\thunderbird\ which is where I put it.

• Manually renamed the profile on C: to something totally different just to be sure and T'bird still accepts the profile on D:

I can't explain it but at least it works now. I'll delete the old copy from C: in a ~day or 3 to free up the space on the SSD.

Thanks for all the help!

Update - It's been 3 days and T'bird continues to use the renamed / edited profile on D. So I guess it's safe to delete the profile on C, which is now out-of-date.

Seems to me, the installer for T'bird (and Firefox) could easily automate the process of migrating a profile and selecting its location during a clean install:

It could ask: Do you want to import a profile? Y/N If Y, allow user to browse for the profile on USB or elsewhere. If N, create new default profile.

Do you want to specify the location for the profile? Y/N If Y, allow user to browse to desired location for the profile. If N, create/migrate the profile selected above to default folder on default drive.

If N/N, create a new profile in default folder on default drive.

My humble 2 cents...

And thanks to everyone for the help here. - Appreciated!