WordPress to HTTPS – Firefox unsafe content

Hello, i switched the site https://skiclub-karlshuld.de/ to https with the following instructions https://www.webongo.de/wordpress-https-umstellen/
So i used the search and replace plugin to change the whole content.
If you open the site with Google Chrome, there is no mistake… but If you open the site with Firefox, it says unsafe content.
I searched the mysql database and replaced all http to https.
The SSL-certificate is working.

Has someone an idea for me, why there is unsafe content?

Regards Max