If you select "TLS if available" Thunderbird will make a TCP/IP connection to the mail server and send a command to ask what capabilities the mail server has. If it says it supports STARTTLS Thunderbird will change the connection to a TLS connection. However, if the mail server doesn't support STARTTLS the connection doesn't fail. Thunderbird continues to use a normal connection. This is a security risk since Thunderbird doesn't display some icon to indicate whether the connection is secure like a browser does, and you're vulnerable to man in the middle attacks (MITM).
Source:
http://kb.mozillazine.org/Secure_connections_-_Thunderbird
