I did a quick check on that domain and it states that firefox.dns.nextdns.io is DOWN. Is Mozilla aware of this?

Many Thanks for any help!

Since you are using macOS 15.0 perhaps your firewall on the OS is blocking Firefox. https://support.mozilla.org/en-US/kb/firefox-blocked-macos-15-sequoia-firewall-what-you

I have been using macOS 15.0 for a week now with NO problems whatsoever. This problem "spontaneously occurred" yesterday, and I cannot link it to any other cause other than FFX not being able to connect to firefox.dns.nextdns.io

Mozilla has partnered with nextdns for quite some time now for secure DNS resolution. I checked about 5 different DNS resolve sites and ALL of them state that firefox.dns.nextdns.io is DOWN. I don't know how to escalate the issue or get to someone who is higher up. It also affected the functionality of my VPN which I now have to keep turned off because of this issue with FFX. Without being able to resolve a domain name it cannot connect to anything.

I was going to recommend filing a bug on bugzilla.mozilla.org but I found out NextDNS has a forum too. They might be able to check their service quicker & confirm the issue.

I found this slightly similar thread but with a different error message: https://help.nextdns.io/t/h7y69lt/firefox-125-0-3-error-ns_error_generate_failure

It did make me wonder though which option you selected, Default Protection or Increased Protection? I thought on a hail mary switching between the two could get it working. But if 5 DNS resolver sites say that domain is down then I really doubt those options will force firefox.dns.nextdns.io to work again.

Another thread to check as well, might give some clues: https://help.nextdns.io/t/q6ym4vl/why-was-i-banned-from-the-service

I'd open a issue/bug in their "Bug Reports" forum just in case they're not aware of it: https://help.nextdns.io/category/bugs