the error occurs because thunderbird needs to be TOLD exactly where the profile is and its name. To avoid this in the future, always copy the thunderbird folder (.....appdata\roaming\thunderbird) and not just the profile (...appdata\roaming\thunderbird\profiles\<profilename> If you fell slightly technical, you can do a quick fix by opening the profiles.ini (simple text file) at appdata\roaming\thuunderbird and edit it to have the correct profile name. save and restart thunderbird . If that scares you, there is a longer set of steps to achieve it. Let us know.

to access that area on hard drive, at desktop enter %appdata% in search window, and that should open the appdata folder from there you will see the thunderbird folder.

Thunderbird doesn't look for profiles in the default location where profile folders are stored. Instead, it uses a profiles.ini file to keep track of the profiles it knows exist and where the corresponding profile folders are located. Thus, merely adding the profile folder to the Profiles folder like you did wouldn't make Thunderbird recognise it. Since that profile folder isn't associated with a profile in profiles.ini, it will simply be ignored. And since you deleted the folder associated with the default profile that Thunderbird wants to use, it gives you the error that the profile cannot be loaded.

You could have avoided that problem by transferring the whole enclosing Thunderbird folder, as david said, but if this is a single profile that we're talking about, you could also solve the problem by giving the profile folder you want to use the same name as the profile folder you deleted, i.e. the folder that Thunderbird is looking for… Is the profile folder you deleted in the Trash so you may get the folder name from there?

If this is a new installation and you don't have profiles stored in profiles.ini that you want to preserve, you may also bypass that blocking error message as follows:

1. Quit Thunderbird if it's running.
2. Delete profiles.ini.
3. Launch Thunderbird. Instead of looking for a profile folder that no longer exists, it will now create a new profiles.ini file with a new default profile and will let you proceed.
4. You will be asked to set up a mail account, but you don't have to. You may cancel the account setup process and choose the option to Use Thunderbird without an email account when asked for confirmation.
5. Choose Help > Troubleshooting Information from the menu bar.
6. Click about:profiles in that page to open the About Profiles built-in Profile Manager.
7. Create a New Profile, read the introductory text if this is new to you, then click Continue.
8. Click Choose Folder and choose the profile folder you brought from the other computer.
9. Name the profile however you want and click Done. You have now associated the profile folder with an actual profile that Thunderbird knows about.
10. The new profile may have already been set as the default. If not, use the Set as default button under it to make it so.
11. Click Launch Profile under the new profile to start using it.

You may see similar instructions to accomplish the same using an external Profile Manager tool. You'd have to use that if you wanted to solve this problem without re-creating profiles.ini, but that's an unnecessary complication in this case because we have no other profiles to preserve here. Both versions of the Profile Manager are documented in the following article:

https://support.mozilla.org/kb/profile-manager-create-and-remove-thunderbird-profiles

Thanks so much, David GG. I have emails from the early 2000's in that profile. I followed your instructions completely and all is well. It was the step by step instructions that work. Until I deleted the profile.ini file, it just wouldn't open. I did attempt to edit the ini file, but it didn't work for me. I'm inexperienced at editing files.