I just checked, there is no simple DNS settings in firefox…
I have no idea what you were doing, maybe you tried to set up a Proxy?
There is DNS over HTTPS setting under Privacy & Security, where you can set up Cloudflare, NextDNS or Custom DNS. This works for me, but if you set this to "Max Protection", a lot of sites will not work, which is not firefox's fault.
You really should set the primary DNS in your router's settings, but if that's not possible (sometimes it's not an option), then the second best is to set it in your device's network settings.
I just checked, there is no simple DNS settings in firefox…
I have no idea what you were doing, maybe you tried to set up a Proxy?
There is DNS over HTTPS setting under Privacy & Security, where you can set up Cloudflare, NextDNS or Custom DNS. This works for me, but if you set this to "Max Protection", a lot of sites will not work, which is not firefox's fault.
You really should set the primary DNS in your router's settings, but if that's not possible (sometimes it's not an option), then the second best is to set it in your device's network settings.