I had configured Ubuntu apache use use self-signed certificate with ssl and https. I followed here https://www.digitalocean.com/community/tutorials/how-to-create-a-self-signed-ssl-certificate-for-apache-in-ubuntu-16-04.
But it seems that it is "Not secure", "Connection is not secure", and "invalid" certificate. Is there the way to create a SSL certificate file to install on browser such as Firefox or Chrome to get rid of "Not secure", "Connection is not secure", and "invalid" or to create a valid one?