Thunderbird doesn't recognize a valid yahoo email password. How to fix it?

Suddenly, I cannot login to my yahoo email account within Thunderbird using my password. I CAN, however, login to yahoo mail outside of Thunderbird with this same password. I restarted Thunderbird several times, rebooted the computer, but nothing helped. Other email accounts (comcast and gmail) within Thunderbird work fine. I will appreciate any help to resolve this, because this is my main email account. THANKS!