Firefox "Server Not Found" (dnsNotFound)

What’s actually happening

Firefox shows its 'Server Not Found' page with code dnsNotFound. The lookup came back empty — no address to connect to. It can be a single dead domain (typo, expired, never existed) or every site if Firefox's own DNS path is blocked. A quick check: if Chrome or another device on the same network loads the same URL, the fault is Firefox-side, often its DNS-over-HTTPS settings.

Common causes

• The hostname is mistyped, or the domain has expired / never existed (true NXDOMAIN)
• The configured DNS resolver is unreachable or not answering
• Firefox's DNS-over-HTTPS (Secure DNS / Trusted Recursive Resolver) is enabled but its DoH provider is blocked or failing, so lookups die only in Firefox
• A firewall, antivirus, or proxy is intercepting or dropping DNS queries from Firefox specifically
• Stale DNS cache or a hosts-file entry pointing the domain nowhere valid

How to fix it

1. Verify the domain actually resolvesRun 'nslookup example.com' or 'dig example.com' in a terminal. No answer / NXDOMAIN means the domain is the problem (check spelling and whether it's expired). A good answer means the fault is local to Firefox or the OS.
2. Toggle Firefox's Secure DNS (DoH)Settings > Privacy & Security > DNS over HTTPS. If a site loads with DoH off, the DoH provider was being blocked — switch providers or set 'Default Protection'. You can also test in about:config with network.trr.mode (0 disables TRR). DoH issues hit Firefox alone, which is why Chrome still works.
3. Flush DNS and check the hosts fileWindows: 'ipconfig /flushdns'; macOS: 'sudo dscacheutil -flushcache; sudo killall -HUP mDNSResponder'. Then review the hosts file (C:\Windows\System32\drivers\etc\hosts or /etc/hosts) for a bad entry pinning the domain.
4. Rule out firewall / AV / proxyTemporarily disable any VPN, security suite's web protection, or system proxy (Settings > Network Settings). If the site loads, that layer was dropping Firefox's queries — add an exception rather than leaving protection off.
5. Test in another browser to localize itOpen the same URL in Chrome or Edge. Loads there but not Firefox = Firefox config (almost always DoH). Fails everywhere = OS resolver or the domain itself; move your fix up the stack accordingly.