This article describes problems where Firefox cannot load websites, but other web browsers (such as Internet ExplorerSafariEpiphany on Gnome or Konqueror on KDE) can. When this happens, Firefox may show a Server not found or Unable to connect error message.

• For other error messages that display when you try to view websites in Firefox, see Websites don't load - troubleshoot and fix error messages.
• If none of the web browsers on your computer can load websites, see Firefox and other browsers can't load websites.

Internet Security software blocking Firefox

If you were able to load websites until you updated Firefox or until your Internet security software was updated, reconfigured or a new one was added, your Internet security software (including firewalls, antivirus programs, anti-spyware programs, and more) is likely preventing Firefox from connecting to the Internet.

In general, you should make sure your Internet security software is up-to-date and remove Firefox from your program's list of trusted or recognized programs, then add it back. For instructions on how to configure some 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 Firefox proxy settings:

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 Advanced panel.
3. Select the Network tab.
4. In the Connection section, click Settings….
5. 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 settings to another browser's (such as Internet Explorer — see Microsoft's guide to proxy settings)(such as Safari — depending on your OS X version, see Mac OS X 10.6: Enter proxy server settings, OS X Lion: Enter proxy server settings, or OS X Mountain Lion: Enter proxy server settings).
6. Close the Connection Settings window.
7. Close the Settings page. Any changes you've made will automatically be saved.

If you find that changes you make to your Firefox connection settings are not remembered when you restart Firefox, see How to fix preferences that won't save.

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 FilterSearch field, type network.dns.disableIPv6.
3. In the list of preferences, double-click network.dns.disableIPv6 to set its value to true.

DNS Prefetching

Firefox attempts to speed up loading new websites by using DNS Prefetching, which can cause page load errors with some system configurations. 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. Right-clickHold down the control key while you click in the list of preferences, select New, and then select Boolean.
3. In the Enter the preference name window, enter network.dns.disablePrefetch and click OK.
4. Select true when prompted to set the value and click OK.

Firefox cannot load certain websites

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

1. Click the menu button to open the menu panel.
2. Click History and select Clear Recent History…
3. In the Time Range to clear: drop-down, select Everything.
4. Below the drop-down menu, select both Cookies and Cache. Make sure other items you want to keep are not selected.
5. Click OKClear Now.

Check for malware

If clearing your cookies and cache did not enable you to load the websites that did not work in Firefox, you should check your computer for malware. Certain types of malware are known to target Firefox and can prevent it from loading various websites:

• If you have an antivirus or Internet security program, update its detections database and do a full scan of your system.
• If you still have problems, follow the instructions in Troubleshoot Firefox issues caused by malware.

Based on information from Error loading websites (mozillaZine KB)