If you copy the Thunderbird folder from the old computer and place it in the same location on the new one, the complete setup is replicated:

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

On Windows, the default location is:

C:\Users\<winusername>\AppData\Roaming\Thunderbird

which is hidden by default in File Explorer.

I have the same problem. I am using windows 10. I have 'View hidden items' selected but cannot find a folder C:\Users\Sylvia\AppData\Roaming\Thunderbird Sylvia has many folders but not AppData. I have looked in the public folder as well but to no avail.

Try this: Windows key + R, type %appdata%, click OK.

Many thanks - I can now see the folder and have put this into my backup plan, but does this just backup the profile or all of the email messages?

All the mail (and address books, calendars, settings, add-ons etc.) is in the profile folder, which is in the ...\Thunderbird\Profiles folder, with a name like 8characterstring.default. You can restore individual items from the profile folder if necessary, but if you need to restore a complete setup from backup or want to transfer a complete setup to another computer, the Thunderbird folder is the best means.

http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Thunderbird

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

Excellent - many thanks that has been most helpfull.