There is an html nesting error on your page:
Within the CF7 form there is another form tag with ..id="contactform" class="comments-form" method="post".. and a closing tag below which should not be there.
The closing tag of that inner second form tag is seen by the browser (FF 43.0.4) as the end of outer CF7 form tag, and due to this the CF7 script can't find the relevant div within its form to output the error/success response.
Note to plugin author: $responseOutput is used without a check for empty in scripts.js