Firefox Server Not Found: compare browsers and check proxy settings
What to do first
First determine whether the failure is specific to Firefox, one website or the wider connection. Record the current configuration before correcting an identified proxy mismatch.
This is a general troubleshooting guide, not an announcement of a current outage.
Affected products and symptoms
- Product
- Mozilla Firefox
- Versions and conditions
- Firefox for desktop on Windows, with another browser available for comparison
- Error codes and identifiers
- Server Not Found
Symptoms
- Firefox displays Server Not Found
- Another browser opens the page but Firefox cannot connect
Causes and conditions
Candidates include an incorrect URL, network or name-resolution trouble, and Firefox proxy configuration. The message alone does not establish a DNS failure.
Before you start
- Record the URL, error text and Firefox version
- On work or school devices, obtain the required proxy configuration
Precautions
- Do not bypass managed settings or a required proxy
- Change one connection setting at a time and restore ineffective trials
Tools and official pages used in this guide
Step-by-step instructions
Check the address hostname for mistakes and retry from an official link. Open another familiar website in the same Firefox window.
Record whether one site or all tested sites fail. Keep certificate warnings and explicit proxy-refusal errors separate.
Record four comparisons: target and known alternative site in Firefox and another browser. Target-only failure across browsers points toward the destination; universal failure toward connectivity; Firefox-only failure toward its configuration. Keep network/account conditions fixed and compare the error hostname with the intended name.
Enter the same URL in another browser on the same PC and network. Keeping those conditions fixed makes the comparison useful.
If several sites fail in both browsers, investigate the network first. Continue with Firefox-specific checks when the other browser succeeds.
Use an official or administrator-confirmed entry instead of blindly retrying an email link. Internal names may require VPN and corporate DNS; public-site success does not prove internal access. Check required VPN and avoid assuming public DNS substitution will help, since it can break internal names. Give administrators matching-name results first.
Official links for this step
If settings cannot change or revert, ask about policy or managed extensions. Do not rewrite configuration files merely to bypass an unavailable control. Confirm the required proxy mode and approved Firefox configuration method for this network.
Open Settings from the Firefox menu. Under General, find the network or proxy section and open its configuration dialog.
In the newer layout, use Privacy and security, then advanced settings under connection/software security, and open proxy configuration. Follow the layout actually shown.
Record the selected mode and any host, port or configuration address. Check whether your network requires the system proxy, manual values or an automatic configuration URL.
Only replace an accidental manual setting with No proxy when you know your personal connection requires none. Use administrator-approved values on managed networks.
Record proxy mode, host, port, exceptions and automatic URL, redacting internal information publicly. Old proxies can remain on personal devices, but organization automatic settings may be intentional. Save originals, change one confirmed value, retest the same two sites and restore it if unhelpful.
Official links for this step
Save the setting, reopen the original URL and test another known site. Compare the results and restore an experimental change that did not help.
If the dialog identifies an extension controlling connectivity, note which one and its purpose. Ask the administrator about an organization-required extension.
Use matching hostnames in DNS comparisons. A resolved address does not establish web-port or proxy connectivity, so open the actual page too. With unresolved names, repeated cookie deletion or certificate changes do not target the demonstrated failure.
If the problem began after a Firefox update, ask the security-product support team or administrator to check its application-access rules.
Include the URL, time, successful comparison and original proxy mode. Do not disable the entire firewall or change DNS settings without a diagnosed reason.
For persistent Firefox-only failure, check update timing and security-product permissions through approved help or administration rather than stopping the entire firewall. Record any controlling extension and whether it is managed. Provide version, hostname, time, four comparisons and original proxy mode. A new certificate error is a separate next-stage failure.
Check the result
- Firefox opens the original URL and another site on the same network
- Connectivity works while preserving the required organizational proxy
- Firefox opens the original URL and a known alternative, with test conditions recorded
If the problem continues
- If all browsers fail, investigate the network or site instead
- For a persistent Firefox-only failure, provide support with the connection configuration and comparisons
Scope of this guide
Troubleshooting guide — Firefox opens the original URL and a known alternative, with test conditions recorded
Frequently asked questions
Should I change DNS immediately?
No cause is established by the message alone. Compare the URL and another browser, then inspect Firefox settings if only Firefox fails.
Why is Network Settings absent?
Firefox layouts differ. Also look under Privacy and security for advanced connection and software-security settings.
Should I reset Firefox if Edge works?
First compare the same URL/network and review proxy mode, managed extensions and post-update permissions. Profile reset affects stored data and settings and need not precede focused connection checks.
Official sources and dates
- Server Not Found - Troubleshoot connection problems ↗
- Connection settings in Firefox ↗
- Websites don't load - troubleshoot and fix error messages ↗
Source publication or resolution date: 2026-07-28. Sources checked: 2026-10-13. The check date is not the date the problem first occurred. Interface labels can vary between versions and display languages.