Did you have exactly the same version of Thunderbird on both the old and the new computer? If no then get them operating on same version.

re: none of the old customem folders show, just the default ones, and even in those there are only 5 very old (2003,2013,2014) emails in the Sent folder. The the old computer has 8971 emails in its Sent folder.

Are you talking about an IMAP account, a POP account or folders in the 'Local Folders' mail account ?

If IMAP On old computer, did you check the Account Settings > Synchronisation & Storage settings to make sure all folders were selected to download full copies of emails and not just headers? Did you then fully synchronise all the folders before Exiting Thunderbird? File > Offline > Download/Sync now

After exiting Thunderbird did you copy the 'Thunderbird' folder located in the 'Roaming' folder?

• C:\\Users\User Name\Appdata\Roaming

On new computer, Exit Thunderbird if running delete the 'Thunderbird' folder from the C:\\Users\User Name\Appdata\'Roaming' folder paste the copied 'Thunderbird' folder into the 'Roaming' folder.

re:Might it help to change that to File per Message before copying to the new system? No.

I have version 91.4.0 installed on both computers. Before posting here I had tried uninstalling and reinstalling TB on the new computer with the same results both times and I'm not certain that the versions were the same th first time, but TB seems to automatically keep itself up to date pretty reliably.

But I just noticed to my surprise that the old computer is running a 32 bit version. It is a 64 bit machine and I thought I had installed 64 bit software when available. Does that matter? If so can it be safely upgraded to the 64 bit version somehow? I don't want to try to do that and find that I can no longer see my email at all. Probably better to install the 32 bit software on the new machine and leave the email archives active on the old.

The email account is POP, so I guess none of the IMAP stuff applies.

I have been copying from and to C:\Users\Leo\AppData\Roaming and visually verifying that the structure and data are identical on the two machines. I have dual monitors so I can see both machines at once by switching the secondary monitor to the old machine.

Thanks for your suggestions.

re :the old computer is running a 32 bit version. It is a 64 bit machine and I thought I had installed 64 bit software when available. Does that matter? If so can it be safely upgraded to the 64 bit version

I used to run a 32bit version of Thunderbird on a 64bit computer. After Exiting Thunderbird - I uninstalled the 32 bit version by deleting the 'Mozilla Thunderbird' folder directly from: C:\\Program Files (x86)\Mozilla Thunderbird

When I downloaded and installed the 64bit version, I noticed it automatically got installed in the 'Program Files (x86)' folder and not the 'Program Files' folder. So, if it happens to you, do not be surprised nor concerned. It is done deliberately by Thunderbird in cases where a previous version used 32bit. This means where the Thunderbird program is installed is not necessarilly a determination of 32 or 64 bit.

The info in 'About Thunderbird' would tell you if you have 32bit or 64bit. Help > About Thunderbird

Thanks for the info as I was wondering about that, but when I noted that the old computer had a 32 bit Thunderbird it was by looking at the "About Thunderbird" screen to double-check for matched version numbers in the two systems and saw that the version numbers matched, but not the bit width. Does that count as a mismatched version?

When I read the instruction on the Mozilla website for transferring to a new computer it seemed to say that the TB folder from the source system should be copied into the TB folder on the destination system instead of on top of it. That is, it says to right click on the TB folder in the source system and copy, then open the TB folder on the new system and paste it into the blank area of the contents listing. This would add a second TB folder inside the original one on the new system. I did not do this as it did not match what I had done in the past, nor did it make sense to me.

For now I am working on verifying that my data was copied correctly into the new system. So far the size of the files on the new system seems to be slightly larger on the old, which makes some sense but was somewhat unexpected.

Thanks for your help.

re :TB folder from the source system should be copied into the TB folder on the destination system instead of on top of it.

There are no instructions anywhere that say the copied 'Thunderbird' folder should be pasted into the default 'Thunderbird' folder.

Instructions: https://support.mozilla.org/en-US/kb/moving-thunderbird-data-to-a-new-computer It states the following:

• From your Thunderbird profile folder in Windows Explorer, go up three levels.
• Right-click inside the folder, then select Paste.
• When prompted, choose to replace your current data.

When you go up three levels you are in the 'Roaming' folder - not the 'Thunderbird' folder. So you are pasting into the 'Roaming' folder. As the folder you are pasting is identical in name to a folder that already exists, you should get prompted what to do and you would choose to replace the current with the pasted.

Personally, I always advise you delete the default 'Thunderbird' folder from the 'Roaming' folder before pasting the copied 'Thunderbird' folder into the 'Roaming' folder. I did mention that in my first comment above.

re : This would add a second TB folder inside the original one on the new system. I did not do this as it did not match what I had done in the past, nor did it make sense to me.

Quite right. It is obvious common sense if you want to create the same setup on another computer then you create the same setup.

I finally solved the problem by copying the Thunderbird folder from the old computer's Local node into the Local node of the new computer. Now it appears to have all of my folders and emails and I was also able to download and organize new emails from the server. I'm not sure why this was necessary, but over the years I have used TB to access newsgroups, imported data from an email file sent by Apple, accessed a second email account for a friend, created lots of folders and sub-folders, etc.

For anyone else doing this, I was able in the end to simply close Thunderbird and copy in the missing data and when I restarted the program everything was in place as on the old PC.