I followed the How-To Geek instructions titled "Get your Yahoo! email in Mozilla Thunderbird for free" including downloading and installing the YPOPS client. Re-booted the PC after installing YPOPS Client, then added a new Yahoo account in Thunderbird per instructions. All went well. My password was accepted and the message "There are no new messages on the server" is displayed for several moments. Except Yahoo.com has 19 read and un-read emails sitting in the Yahoo server's in-box.