Created attachment 1906 
That is how my TB 2.0.0.17 understands "--compose"

When Thunderbird is not running the helper starts it with a command like this:

/usr/bin/thunderbird --compose to='devnull@example.com,cc='',attachment=''

Thunderbird doesn't understand the "--compose" and treats the comma separated terms as subsequent mail addresses (check out the screenshot).

The proper command line option is "-compose" (single dash). The patch fixes the problem.