here are steps: Here is the link for moving a profile in Windows when installed in default location:

https://support.mozilla.org/en-US/kb/moving-thunderbird-data-to-a-new-computer

here is the info in brief steps:

1. ensure thunderbird is NOT running on old machine
2. enter %appdata% in search window on task bar and press Enter key
3. this should open windows file explorer in Appdata\roaming folder
4. highlight the Thunderbird folder and copy to external media
5. ensure thunderbird is installed on second computer and NOT running
6. enter %appdata% in search window on task bar and press Enter key
7. you should be in Appdata\roaming folder , copy&paste the exported Thunderbird folder there. This may prompt to overwrite an existing folder of same name. allow it
8. start thunderbird and all should be there

That is, don't just copy the profile, copy the thunderbird profile and do it when thunderbird is not running.

That is, this works if you downloaded thunderbird from thunderbird.net. If you downloaded from Microsoft, these steps don't work

Thank you. I tried (several times) downloading from TB site, and replacing the folder as you described. The results were the same...

My guess is that you are not overlaying the existing thunderbird folder but may be pasting the thunderbird folder to appdata\roaming\thunderbird which doesn't work. be sure you paste to ...appdata\roaming You can check this by clicking help>troubleshootinginformation and scrolling to 'profile folder' and clicking 'open folder.' You should now see the profile you pasted in.

Thank you, David! It appeared that my old TB folder was created by an old TB version. Since then the formats have changed, and to just replace the TB folder with the old one could not work. I found a TB version compatible with my files, but apparently the mail repository was corrupted (the old PC is not bootable; I copied folders from it using Ubuntu). So I had to satisfy myself with just converting the old address book (.mab) to the the new format (sqlite). Best, K.