Error message that I received, "Sending of username did not succeed. Mail server pop.webbox.com responded: Invalid userid/password"

sounds like an anti virus issue to me.

Restart the operating system in safe mode with Networking. This loads only the very basics needed to start your computer while enabling an Internet connection. Click on your operating system for instructions on how to start in safe mode: Windows 8, Windows 7, Windows Vista, Windows XP, OSX

If safe mode for the operating system fixes the issue, there's other software in your computer that's causing problems. Possibilities include but not limited to: AV scanning, virus/malware, background downloads such as program updates.

The Safe Mode solved the problem and enabled me to download my eMails. I ran my antiVirus (Avast), Windows Defender, CCleaner, Malicious Software Removal Tool all with the same results of no corrupt or malicious programs found. The only recent program that I have added in the past couple of months was Adobe Reader(latest version 12, I think). I uninstalled that along with its addon for my browser (PaleMoon). No Change! Years ago I turned off many unnecessary programs, mostly via msconfig. I have tried to prevent all unnecessary programs from phoning home. I have a program called Startup Monitor which keeps vigilance over my pc. Nothing is allowed to load at start up w/o my express permission. Any further suggestions before I reformat my hard drive? Thank you for the safe mode advice...good call!

re: The Safe Mode solved the problem and enabled me to download my eMails.

If this was computer safe mode then another program is causing a conflict. If it was Thunderbird safe mode (restart with addons disabled), then an addon/extension is causing the conflict.

CCleaner has a nasty habit of removing passwords etc, you may need to make sure it does not include anything to do with Thunderbird. It may be this program that removed the password and so would cause the error you experience.

Assuming CCleaner has removed the Thunderbird stored password, do the following. Stop CCleaner from scanning for Thunderbird passwords.

then in Thunderbird: Tools > options > Security > Passwords tab click on 'Saved Passwords' click on 'show Passwords'

locate mailbox and smtp account passwords. select those lines and click on 'Remove' then click on 'close' then click on 'OK' to save changes to Options.

Close Thunderbird, allow a small amount of time for it to close fully. Restart Thunderbird. at the prompt enter the correct password and select to remember the password, click on OK.

Windows Defender, CCleaner, Malicious Software Removal Tool, etc never run in my background, only when I manually activate them. The only add-on I have is Lightning 2.6 (calender). I disabled it and still received the error message. I did locate mailbox and smtp account passwords and removed them, and followed your directions. Prior to I re-entering the passwords, the original error message appeared. Bummer!

The fact that Safe mode on computer allowed you access to emails proves that it is another program on your computer causing the problem.

I mention CCleaner because it is known to cause issues with Thunderbird such as clearing the passwords and the session info and you are using that program.

You stated that you had run CCleaner manually, but you did not say whether you had gone into CCleaner and excluded Thunderbird to stop it causing problems.

re: Prior to I re-entering the passwords, the original error message appeared.

because at that point there is no password in thunderbird, so you will get that error message.

You would then have clicked on OK and the prompt should appear for the password, did this happen, did you enter the password and select to remember it? Is there a new stored password in Thunderbird - see previous instructions for removing the password - as you removed them there was no password, so you entered the password at the prompt now recheck, is the password stored correctly ?

I had not excluded Thunderbird in CCleaner. In order to do that, I would have to turn it on again....however, I did manually remove the passwords in Thunderbird, and then manually re-entered the passwords when the program requested them upon Thunderbird's startup(those passwords are now re-stored in the Thunderbird Password section)....also, I had stored in the "Tools > options > Security > Passwords tab click on 'Saved Passwords' click on 'show Passwords'" location, other old, discontinued email addresses (3 of them). All had their passwords still listed, so I suspect that CCleaner didn't remove any of them.

Thank you for your good suggestions!