No, you made an error in placing it. here are the steps:

BACKUP

1. ensure thunderbird is not running
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

RESTORE

1. ensure thunderbird is installed and NOT running
2. enter %appdata% in search window on task bar and press Enter key
3. you should be in Appdata\roaming folder , copy&paste the exported Thunderbird folder there. This will prompt to overwrite an existing folder of same name. allow it
4. start thunderbird and all should be there

Hi David, Thank you for your reply. I believe I followed your steps but I still get the same error when I use the source Thunderbird file in the new computer. The error is "Your profile cannot be loaded. It may be mission or inaccessible" The source Thunderbird is version 140.8.0esr which is different from the new computer version which is 148.0. Both computers claim to be up to date. Any ideas on what to try? Thanks,

steve

See step 1. pres alt+ctrl+delete open the task manager and look through the list for any instance of Thunderbird, especially as a background process and if it does not close, kill it.

While Davids approach generally works with older versions of Thunderbird I am finding that it astruggles with the profile per install and other "enhancements" that have delivered nothing meaningful to the general user. So I have my own way that mostly works.

On you new computer open the %appdata% location and delete the Thunderbird folder found there. That restores everything to just installed default.

On your old computer open Thunderbird and then open the troubleshooting information on the help menu. Locate the Profile Folder entry in the table and click the Open Folder button. Now I personally suggest having a cup of coffee to sort of make sure Thunderbird gets new mail in this instance and things should be fairly stable because of the time delay. CLose down Thunderbird. In the open file explorer window select any file of folder showing and press Ctrl+A that will select everything in the current folder and any sub folders and either drag it to a USB drive or copy and paste to one.

Again toddle off for more coffee (you will never sleep again) but Windows and all modern operating system report files being copies long before the process completes. THere is no harm no foul mostly, but if you are unplugging the device to move to another machine there is nothing more likely to cause total fa9ilure that an incomplete disk write. (Keep in mind that your profile may be quite large so I would suggest you have 100Gb free on the drive before you start just to make sure is does not run out of space. I would expect even a very large profile to be smaller that 40Gb but on this occasion I am a fan of overkill. Especially after some of the horror stories about profile sizes caused by antivirus products I have seen recently with single folders in the hundreds of GBs) On your new computer start Thunderbird. It will want to add a new account, just cancel out of that until you get to a point where you can access the profile button in the troubleshooting information. open the profile from the troubleshooting information and again close Thunderbird.

Copy the files from your USB drive to the opened profile folder and expect to be prompted to replace files. That is how you know you are copying to the right location.

Start Thunderbird and it should "just work"

One major caveat. If you got your copy of Thunderbird from the Windows store everything I or David has said is mute. Personally I refuse to support the store version. There are to many bugs and not enough documentation from Mozilla on how their product is supposed to work in that constrained environment. See https://bugzilla.mozilla.org/buglist.cgi?quicksearch=msix for bugs that are marked as involving the Microsoft store installer

One obvious bug, and difference, is you can not open the profile folder from the troubleshooting information if you have the store version.

Matt is correct; my approach usually works, but never for MS versions of thunderbird. You are getting that error because your appdata\roaming\thunderbird folder is corrupting the installed version. Here are my alternate steps that are mostly the same as Matt's, although I prefer a shot of bourbon to carry me through:

1. start thunderbird on old PC
2. click tools>export and then click 'open profile folder'
3. exit thunderbird - you should now be in the profile folder. You can verify that by looking to see if abook.sqlite is there.
4. copy profile content to external media
5. install thunderbird on new pc, downloaded from thunderbird.net
6. start thunderbird
7. click help>troubleshootinginformation
8. scroll to 'profile folder' and click 'open folder'
9. exit thunderbird
10. delete everthing there
11. now, paste contents of exported profile
12. if there is a compatibility.ini file, delete it
13. restart thunderbird