re :Is there a problem with this kind of copy?

No. As I understand it, the user already had Thunderbird installed and had created a mail account and downloaded a few recent emails.

then you did one of the following: Either: You copied the profile folder name 'contents' from the older machine and pasted them into the new profile folder name on new computer.

this would completely overwrite anything previously in that profile. So deleting the mail account and any emails.

OR You copy pasted the Appdata\Roaming\Thunderbird folder which contained your Profiles folder and 'profile.ini' file, thus overwriting everything on the Win8 machine.

You would need to be very explicit in explaining exactly what you did and exactly which files or folders you copied. It would also be useful if you provided images showing showing contents of the Profile. What is in the Appdata\Roaming\Thunderbird\Profiles folder , this will confirm whether you've got more than one profile folder name.

Pop mail accounts usually download from server and delete the copy off the server. So the only copy is on your computer. This may explain why the newer emails did not redownload.

If you logon to the webmail account via a browser, are those emails still on the server? Is this a pop gmail account?

In thunderbird you can change the settings in the Pop acccount settings to leave a copy on the server. But remember, you will need to monitor the webmail account to make sure it doesn't reach it's quota limit.

'Tools' > 'Account Settings' or right click on mail account name in folder pane and select 'Settings

then select: 'Server 'Settings' for the pop mail account select 'Leave messages on server' click on OK

I wasn't clear enough.

The user was using Thunderbird on Win7.... Then, we installed a brand new PC for him, with a recent installation of Win8. We copied the profile and then installed Thunderbird.

The account is from a corporative server (not Gmail). And the e-mails are not in the server anymore.

Is not Thunderbird saving the e-mails in another folder?

re; win 8 machine. Was Thunderbird already installed and the had the user already created mail accounts and had downloaded emails onto the win8 machine ? Are these the emails that are missing?

I know you copied the win7 thunderbird profile, but I need to know exactly what you did and what you copied as this would effect the result.

So what folder did you copy - it's name. what process did you use ?

You would need to be very explicit in explaining exactly what you did and exactly which files or folders you copied.

Did you copy the win7 profile folder name contents from the older machine and pasted the contents into the new default profile folder name on win8computer overwriting all the files and folders in the default profile?

OR Did you copy pasted the Appdata\Roaming\Thunderbird folder which contained your Profiles folder and 'profile.ini' file ?

OR Did you copy paste just the win7 default profile folder name and put it in the win8 thunderbird 'Profiles' folder so that it contained two profiles?

Please provide images of followng: open the 'Profiles' folder - how many profiles are listed ? Please show image.

open the profiles.ini file using Notepad and copy paste the info or post image.

The copied folder is the Appdata\Roaming\Thunderbird. (It's the second option that you listed). The copy was done before installing Thunderbird.

By the way, the user hasn't created a account, because Thunderbird was not installed yet.

I'm gonna take the images that you are asking for!