It 'sounds like' you're setting up an account that already exists. More info is needed here. Thanks.

Yes it does and yet no accounts are listed on the account settings screen.

How do I remove an account that I can't see?

Okay, so I understand, you're stating that in tools>accountsettings there is nothing there? What you might try is clicking help>moretroubleshootinginformation, scroll down to 'Profiles', click 'about:profiles', and then browse the Mail folder and the ImapMail folder to see if either has anything in it. From your first post, I'm guessing that one of those folders may have an account listed. And, if you feel that, even if there is, it's not right, then my next suggestion is to do a full ininstall and start over.

I have a parallel problem though mine may be self-inflicted. When I installed newest version from mozilla.org this a.m., I had another, unpopulated version of T-bird installed from 2014 that neither I nor Centurylink "tech support" (feckless and insufficiently-trained+intrinsically-incurious) could make work. It lay dormant on my HD in my hopes I'd find someone who could cobble together a config that would work to deliver me a POP3 connection to that account.

When I installed new v, I was pleased to see an auto-detect that harvested Centurylink's settings. Pressing [Done], however, led to the dialog box w/the error msg "Incoming Server already Exists". I can see from other board queries on it it doesn't *necessarily* mean there's a collision w/a previous install, but in my not-Mozilla but general software experience, an error message like that *could well* mean that.

the About Profiles [Open Folder] Mail folder exists, but its subfolders are at 0K or 2K.

I sure would love to get a Thunderbird POP3 connection fully-functional for mail (and even just somewhat for address book).

¿Ideas beyond uninstall-->re-install? I could do that, but I'm looking to mitigate the consequences of an earlier version's possible disruptions.

I would first just shutdown and restart. IF your install overlayed the 2014 profile, there may be several incompatible items, since that's a long stretch. We can work at it. My suggestion if restarting doesn't open a window is to delete the account completely and start over with it. No need to uninstall Thunderbird. When setting up the account, I suggest that you NOT include password until account is defined.

Thank you, and especially the no-password-until-connect protocol (which I would never have thought of, David).

Going to try in the next 30m.

Significant, partial success.

    1. Restarted, tested without password, succeeded, entered password. 
    2. Thunderbird brought in 47 messages (from about 3,100) from 
         my parent Inbox folder, then...
    3. Threw up an error...pwd not recognized (though clearly at one point
        it was recognized enough to let the client grab 47 messages. This smells 
        like a Centurylink defect, but you'd know better than I.
    4. Re-entered password, and no error, but nothing "new" to download. 
    5. Tried to test password by within Thunderbird creating a test message
        to a friend and when attempted to send, got the security error 
        window pictured in the image.

Thanks again