If you have a problem connecting to a website, you may see error messages like Server Not Found. This article describes how to troubleshoot problems that cause this error.

• For troubleshooting other error messages, see Error loading websites.

Try another browser

To begin, try opening a website in another browser (such as Microsoft EdgeSafariEpiphany or Chromium).

• If all other browsers cannot load the website, move on to the No browsers can load websites section.
• If the other browsers can load the website, skip ahead to the Firefox cannot load websites but other browsers can section.

No browsers can load websites

If neither Firefox nor your other browser can load websites, your problem is not in Firefox, so you should seek help elsewhere. Apple support includes these articles: Microsoft support includes these articles:

You should also check the following:You should begin by checking the following:

• Make sure your modem and/or router are both on and are not indicating errors.
• If you are using a wireless connection, make sure that you are connected to the right access point.
• Make sure your Internet security software (including firewalls, antivirus programs, anti-spyware programs, and more) is not blocking connections to the Internet. For instructions on how to configure these programs, see the Configure firewalls so that Firefox can access the Internet article.
• If you use a proxy server, make sure that the proxy server can connect to the Internet. For instructions on configuring proxy settings in Firefox, see Firefox connection settings below.

Firefox cannot load websites but other browsers can

If Firefox cannot load websites, but your other browser can, follow the instructions below.

Cannot load websites after updating Firefox

If you were able to load websites until you updated Firefox, your Internet security software (including firewalls, antivirus programs, anti-spyware programs, and more) is likely preventing Firefox from connecting to the Internet. Some Internet security programs can block Internet access even when they are in a “disabled” state.

In general, you should remove Firefox from your program's list of trusted or recognized programs, then add it back. For instructions on how to configure these programs, see the Configure firewalls so that Firefox can access the Internet article.

Firefox connection settings

If you connect to the Internet through a proxy server that is having connection problems, you will not be able to load websites. To check your connection settings in Firefox:

1. In the Menu bar at the top of the screen, click Firefox and then select Preferences or Settings, depending on your macOS version.Click the menu button and select Settings.
2. Select the General panel.
3. Scroll down to Network Settings and click Settings…
4. Change your proxy settings:
   • If you don't connect to the Internet through a proxy (or don't know whether you connect through a proxy), select No Proxy.
   • If you connect to the Internet through a proxy, compare Firefox's connection settings to your system settings. In Microsoft Edge, you can go to Settings → System and Performance (or, in Google Chrome, to Settings → System) and select Open your computer’s proxy settings to view your system settings. The Apple Support articles Set up a proxy server using Safari on Mac and Change proxy settings in Network preferences on Mac will show you how to view your system settings.
5. Click OK to close the Connection Settings window.
6. Close the Settings page. Any changes you've made will automatically be saved.

DNS Prefetching

DNS Prefetching is a technique Firefox uses to speed up loading new websites. To disable DNS Prefetching:

1. Type about:config in the address bar and press EnterReturn.
   A warning page may appear. Click Accept the Risk and Continue to go to the about:config page.
2. In the Search field at the top of the page, enter network.dns.disablePrefetch.
3. If the preference does not already exist, select Boolean and click the Add button to create the preference.
4. Change the preference value from false to true by pressing the Toggle button.

IPv6

Firefox supports IPv6 by default, which may cause connection problems on certain systems. To disable IPv6 in Firefox:

1. Type about:config in the address bar and press EnterReturn.
   A warning page may appear. Click Accept the Risk and Continue to go to the about:config page.
2. In the Search field, enter network.dns.disableIPv6.
3. In the list of preferences, click the Toggle button next to network.dns.disableIPv6 to change its value from false to true.

Firefox cannot load certain websites

If you find that Firefox can load some websites but not others, first clear your Firefox cookies and cache:

Based on information from Error loading websites (mozillaZine KB)