I think it may make a difference if you use pop.netzero.com for the incoming POP server (on port 995, SSL/TLS security, normal password authentication, full email address for the user name). Make sure TB didn't try to use POP settings for an IMAP account.

http://help.netzero.net/nzhelp/support-home-page/service-help/email/email-setup/

I had tried doing that as well, and it didn't make a difference

Maybe you first have to enable POP access for the account on the netzero website?

which I have done, by having their megamail system. I had been receiving the email through windows mail (no longer supported or downloadable) -- computer crashed and I got a new computer a couple of days ago.

Make sure you don't have an obsolete password in Tools/Options/Security/Passwords/Saved Passwords, Show passwords, and temporarily disable your AV program while setting up the account as these often interfere with SSL connections.

it is the valid password. I tried by putting it in manually on the netzero site (more than once to check that I wasn't doing something in error), as well as putting it in on windows 10 mail program.

not sure where to find tools that would show tools that you refer to (I am not using firefox if that is where I would find tools, I am using chrome control panel has nothing labeled tools -- administrative tools doesn't have something that parallels what you are saying

In TB, press the Alt key to show the Menu Bar, then Tools/Options. Or, click on the 3-bar icon on the Mail Toolbar.

found it...password there is the correct password -- cleared it and reentered it, same results