By default, Windows hides certain files and folders, including the AppData folder where Thunderbird stores your data. To switch that off, try the steps in this article (assuming they still work in Windows 10):

http://windows.microsoft.com/en-us/windows/show-hidden-files

I'm not sure of the safest way to move your data over, but hopefully these articles will get you started on tracking down the data:

• Profiles - Where Thunderbird stores your messages and other user data
• http://kb.mozillazine.org/Files_and_folders_in_the_profile_-_Thunderbird

Unable to find files referred to. Attached 2 images of files displayed within "Mozilla Thunderbird" folder as found reading old HDD with USB Hdd reader. I have set the system to display hidden files in W10 but wonder if this applies in my scenario?

You're looking at the wrong place. Check this article for more information about the profile location. http://kb.mozillazine.org/Profile_folder_-_Thunderbird

You're looking under

D:\Program Files (x86)\?

No, your data is under this path:

D:\Users\username\AppData\Roaming\Thunderbird\Profiles

Please re-read the two help articles for assistance in finding your data.

Thank you , I've been trying for hours....solved at a stroke!