I had Thunderbird configured with an iCloud (me) email account and it worked fine until about two weeks ago. When I tried to send a message, it asked me for the server password. When I entered it, I got a message saying the login or password was wrong. I also had this same problem on another machine with another account. I was able to add and use a Gmail account without any problems.
My setting are: Incoming
Protocol: IMAP Server: imap.mail.me.com Port: 993 SSL: SSL/TLS Authentication: Normal password
Protocol: SMTP Server: smtp.mail.me.com Port: 587 SSL: STARTTLS Authentication: Normal password
I got the settings from How can I configure iCloud mail in Mozilla Thunderbird?
I am able to login to both iCloud accounts through a browser.