MOZILLA_PKIX_ERROR_SELF_SIGNED_CERT error but not allowed to add exception

If a website has a self-signed certificate then Firefox 88 may halt the user from using the site, producing the error MOZILLA_PKIX_ERROR_SELF_SIGNED_CERT without the previously usual option to circumvent it by adding an exception. Maybe it is an "user friendly" change.

An exception can be added anyway by copying the domain name and then going to:
Tools > Options > Advanced : Encryption: Certificates - View Certificates > Servers.

Then click on Add an exception... and fill in the domain name and add it there anyway and exit the settings.

Reload the page with F5.

Reference: https://support.mozilla.org/en-US/questions/944748