Just updated to Firefox 91 and looking in the ‘whats new’ section which contains the following;
HTTPS-First Policy: Firefox Private Browsing windows now attempt to make all connections to websites secure, and fall back to insecure connections only when websites do not support it. Learn more.
Clicking on the ‘learn more’ link reveals this:
HTTPS-Only Mode Alert
Secure Connection Not Available
You’ve enabled HTTPS-Only Mode for enhanced security, and a HTTPS version of blog.mozilla.org is not available.
Learn More…
Maybe this is a roundabout way of proving to you that the system works!!