Access advanced Firefox settings: about:config
Set network.trr.mode from 0 to 2.
Set network.trr.uri to https://mozilla.cloudflare-dns.com/dns-query.
network.trr.mode is a configuration setting in Firefox that controls the use of DNS over HTTPS (DoH). TRR stands for Trusted Recursive Resolver and refers to the use of DoH to send DNS queries over an encrypted HTTPS connection instead of traditional, unencrypted DNS queries.
0 – DoH is disabled:
Firefox uses only traditional, unencrypted DNS (over UDP or TCP) and sends no DNS queries over HTTPS.
1 – DoH is used if normal DNS fails:
If the traditional DNS resolution fails, DoH is used as a fallback.
2 – DoH is preferred, but traditional DNS is used as a fallback:
Firefox primarily uses DoH for DNS queries. If resolution over DoH is not possible, Firefox falls back to traditional DNS.
3 – DoH only:
Firefox uses DNS over HTTPS exclusively for DNS resolutions. Traditional DNS is ignored. This offers the highest security and privacy, but can cause problems in networks where DoH is blocked or restricted.
4 – Traditional DNS only, but can perform DoH queries:
In this mode, Firefox normally sends standard DNS queries but can use DoH in certain cases or tests.
5 – DoH completely disabled:
DoH is fully disabled and cannot be enabled by any configuration or setting.
On the following page you can verify the change: https://one.one.one.one/help/