If you are sure that you eliminated the stored passwords in Thunderbird then the password is usually being supplied by another password vault software or password saving software. Thunderbird is good but it not so good that it can supply passwords that are not listed on the Saved Password screen.

I did follow the instructions to remove all stored passwords through Thunderbird. When I go back to look for saved passwords, the dialog box is empty. Do you have any suggestions for finding where the password may still be stored or Is there a way around this? I tried to uninstall then re-install. All of my previous information loaded rather than prompting me to set up a new account.

I have no idea what software you might be running on your computer. Some antivirus software offers to remember your passwords for you as does some other software. Do you ever get prompted by some application if you want it to remember the password? That would be a hint.

You are just wasting time reinstalling Thunderbird. Even if the passwords were still stored in Thunderbird you personal settings are separate from the program files and would be picked up and used by the new install.