In your header you have the line `<link rel="SHORTCUT ICON" href="http://www.tanzaniabirds.net/favicon.ico">`, telling Firefox to load the favicon via http. Firefox detects these kind of http subresources on a https site as "mixed content" and warns the user about them. You can read more about that [https://developer.mozilla.org/en-US/docs/Web/Security/Mixed_content here]. The easiest solution would be to just update all subresources on your site to use https. Alternatively, you can also instruct Firefox to automatically "upgrade" those subresources to https through the [https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/upgrade-insecure-requests upgrade-insecure-requests content security policy].

The favicon.ico href needs to be changed to "https".