Privacy error in Mozila Firefox: Your connection is not private” (NET::ERR_CERT_AUTHORITY_INVALID).

HSTS stands for HTTP Strict Transport Security. It is a WEB security policy that forces browsers to interact with websites with HTTPs connections.
It prevents protocol downgrades & cookie hijacking.

1. Click on Help menu in menu bar(you can make menu bar visible by pressing ALT+H button)
2.Click on troubleshooting information
3. And click on refresh Firefox
4. Close & reopen firefox

It will refresh Firefox to default settings & issue will be solved

Other alternate procedure

1)enter about:support in address bar & press enter
2) click on Refresh Firefox
3)close Firefox & reopen