I'm not sure that I understand why this is a problem. Thunderbird is a mail client. Firefox is a web browser...most (if not all) web browsers handle the download when you click on an ftp:// URL. Thunderbird's behavior is to call the web browser to do just that.
The above is true, yet the question remains, his email client is not opening the browser, it is opening a composer window. His thunderbird is not understanding the "ftp://";.. It is using it as a "mailto:";
