This error is occurring because there is mixed content (https in the address bar and some content like an image has just http). To resolve it:
HOLD CTRL+SHIFT+J in Firefox to see browser console and search for 'mix' to see what is causing the mixed content error

Find out what is causing the content and remove it
It may be in a Formatted Text & Images part. It may be on the template or layout. Or it may even be in the stylesheet. You can always use F12 on the element to see where it might be located