This is not a new behaviour, this is in effect since Firefox 23. Only passive content is allowed through unsecured channels (i.e. http) if you are using a secure (i.e. HTTPS) connection. This should be fixed at the website.

For more information, you can read this article: https://developer.mozilla.org/en-US/docs/Web/Security/Mixed_content/How_to_fix_website_with_mixed_content

page is running httpprotocol, everything works ok, except woff2 fonts get flagged as Blocked loading mixed active content even tho those too are being served from same server with http protocol

Can you provide the URL with the issue?

one of pages is www.delimano.sk