I have installed Let's Encrypt's SSL certificate for my website using Certbot (CentOS 7). Every major web browser successfully makes https connection with my web server, except Mozilla Firefox. This is what i get as an warning in Mozilla Firefox when trying to load my web site:
vozilo.rs uses an invalid security certificate. The certificate is not trusted because the issuer certificate is unknown. The server might not be sending the appropriate intermediate certificates. An additional root certificate may need to be imported. Error code: SEC_ERROR_UNKNOWN_ISSUER
Here is SSL Shopper's report.
I've been searching past few days for a way to solve this problem, but wasn't able to found solution. What am i supposed to do in order to make Firefox to load my web site without SSL warning?