HTTPS gives a security warning using this plugin

I use this on a site I made for a friend.

https://spiel-der-wandlung.eu/ausbildungen/

It is a HTTPS site, and in Firefox as soon as I scroll down the plugin comes in and gives a security warning. (the lock changes to a warning triangle).

Can this be fixed?

http://ww.wp.xz.cn/plugins/dynamic-to-top/