hi stefano
thanks for your prompt reply.
the form is intergrated in the footer widget the standard {subscription_form} way on http://www.thurartmanufacture.ch. this is, where the error happens.
i've done some further testing and figured out, that in firefox 50.1.0 both field types (drop down and text input) are not working fine:
http://www.biaggi.ch/forumlinks/input_nok.jpg
but on chrome 55.0, the text seems to be fine on input text fields, but still not on the drop down menue: http://www.biaggi.ch/forumlinks/input_nok_chrome.jpg
thanks and regards
silvio