nonius.io uses an invalid security certificate.
The certificate is not trusted because it is self-signed.
The certificate is not valid for the name nonius.io.
Error code: SEC_ERROR_UNKNOWN_ISSUER
It's not even possible to add a security exception:
This site uses HTTP Strict Transport Security (HSTS) to specify that Firefox may only connect to it securely. As a result, it is not possible to add an exception for this certificate.
The website https://nonius.io is not working, FireFox reports this:
It's not even possible to add a security exception: