You asked Firefox to connect securely to selfsigned.org.example. The server sent you credentials that look fine, but have not been verified by a third party. We have no proof that they are who they claim to be.
According to Herdict, other people who visit this site get the same credential. That means this is normal.
You should not give this website your credit card number.
You should not give this website information about accounts you have on other sites.
But it's probably safe to visit.
Firefox will warn you if anything changes in the future.
This shows a nicely formatted view of the certificate, with the parts that are suspicious highlighted.
In this case, instead of the exception dialog, this area gives another link to the website.