Starting with Firefox 62, Mozilla Firefox browser added support for DNS over HTTPS (encrypted DNS). We don't recommend to use it if you already make use of Simple DNSCrypt or other DNSCrypt/DNS over TLS tools, since it would be redundant.  

Note: this Firefox feature is useful only when you are not connected to the VPN. For example, it would help to access our main site (vpn.ac) instead of mirrors which are also getting blocked regularly. It also applies to many websites that are blocked and makes it much harder for ISPs and 3rd parties to see what websites you visit. 

To enable it, open Firefox's Options > Network Settings > scroll down and enable the option "Enable DNS over HTTPS". Leave the option "Use default (https://mozilla.cloudflare-dns.com/dns-query)" checked or alternatively use the following "https://dns.quad9.net/dns-query" 
Test with both and see which one works best for you.