plugin causes Firefox active content blocked warning on SSL pages

[mgason]

I cam across this problem on my live store so I I setup a test site using clean install of WordPress 4.5.3 and the MailChimp signup widget plugin 'Version 1.4.5 | By MailChimp and Crowd Favorite' The site also has woocommerce 2.6.1 as those are the pages using my SSL cert. There are no other plugins.

If you go here with the Firefox browser https://markgason.com/cart/
you will see in the address bar an ! on the secure lock in the address bar warning that Firefox has blocked insecure active content.
Following the instructions for trouble shooting this issue (https://developer.mozilla.org/en-US/docs/Web/Security/Mixed_content/How_to_fix_website_with_mixed_content) go to the Firefox menu Tools/Web Developer/Web Console. You will see 1 warning only.
The URL that is causing the problem displayed in red contains 'mcsf' this is MailChimp code.
I checked my page source code and the Url there is https. The error must come from elsewhere such as the plugin. The plugin contains references to 'mcsf'
Do you have any ideas on how to get rid of this error. It is the only one on my secure pages. It puts customers off using my site.