doesn´t work on firefox

hi there,
strange problem. stb works on safari, chrome and ie very well, but doesn´t show anything besides the overlay in firefox (on mac and windows).

see here: http://einfachgemacht.fluvius-sanus.de

after 50% stb will be displayed on top (this works) press the button (exklusive praxis tipps) and it should be displayed another stb with a getresponse subscription form.

btw: the same button which appears on the top, is implemented at the bottom the site..also doesn`t work

strange, that (in firefox) a youtube video will be displayed perfect. you can check menu>wissen>einfachgemacht tv

hope to get any idea.
thx in advance!

https://wordpress.org/plugins/scroll-triggered-boxes/