I decided to completely uninstall it and start anew....only to discover when I reinstalled it that everything was still there!!! UNHEARD OF!!!

Thunderbird saves personal information such as messages, passwords and user preferences in a set of files called a "profile", which is stored in a separate location from the Thunderbird program files. The program files are static and do not change (except when you upgrade the Thunderbird application version). Because we store profile and program files separately, you can uninstall Thunderbird without losing your messages and settings, and if something goes wrong with a Thunderbird update your information will still be available. It also means that you don't have to reinstall Thunderbird in order to delete your information or troubleshoot a problem.

Two problems: (1) This is for Win 7 & 8, and I have Win 10.

I'm certain with a quick Google search you'll get plenty of hits. Or ask in a Microsoft forum.

(2) Where do I find "folder: C:\Users\*username*\AppData\Roaming\Thunderbird"

Use Windows Explorer. You already have the full path.

Where do I find ... "to enable "Show Hidden Files, to see the AppData folder"?

See 1) above.

Thank you chris1 and the others who so kindly and patiently helped. It's been very difficult for me to follow the instructions given to achieve my goal and had to search the net a lot, but I finally was able to get it done. I've never had to make these major changes before, and 2 at the same time!  :(

Now, I have a nice and neat 2-account profiles and ready to take the next step. May I impose on the experts here to ask:

I made my first address a Default, but the Manage Identities shows only one address for each Setting, separately, and each one is called "Default"...  :) Is this right or should both appear together?

Finally I'll try to import all my files and folders and everything else from WLMail, and do hope I won't get lost in a sea of labyrinths again, lol!

Thanks again to all. Adela

Identities are a way to attach multiple email addresses to one account. You probably don't want this; most users never use or need it. You should probably set up your two addresses up as two separate accounts and forget about the "manage identities" feature.

By way of illustration, I use one account this way. I subscribed to a Spam reporting service that awarded me a unique email address to use when sending spam emails. It is unusual in that I can send to this address, it is "mine" in that it is unique to me, but it doesn't have an Inbox; no-one can send messages to me on this account. However it did seem sensible to me to also use this as the "from:" address when reporting spam. In Thunderbird you can only send "from" accounts set up in Thunderbird as incoming accounts, and this one cannot be set up as an incoming account. But I can add it as an alternative identity on an existing account.

Another use is for those who are able to create unique addresses for use with particular correspondents. You might not want to go through the process of setting up each one as a separate account, and indeed your server may not distinguish between them, simply treating all the unique addresses as aliases of the principal account. Again, the Identity Manager could be an appropriate way to add these to an existing account.

If you have read this far, I think you will agree that neither of these examples is of use or interest to the majority of email users.