Windows XP and Vista provide a built-in firewall component, which controls the programs that can access the Internet and the types of connections that they can make. If Windows Firewall doesn't permit Firefox to make connections, Firefox generates a "Server not found" error when you try to browse to web sites.

Note: This article applies to Windows XP and Windows Vista only.

Checking for Windows Firewall

To see if you're running Windows Firewall:

1. Open the Windows Control Panel.
   • (Windows Vista) Click the Windows icon, and select Control Panel.
   • (Windows XP) Open the Windows Start Menu, and select Control Panel.
2. In the Control Panel, open the Windows Security Center.
   • (Windows Vista) Under the Security header, click Check this computer's security status.
3. Open the Windows Firewall header. Try to locate the following text:
   Windows Firewall is actively protecting your computer
4. If it is set to On (Recommended), you are using Windows Firewall.

Configuring Windows Firewall

If you're using Windows Firewall, you must verify that it is properly configured.

1. In the Windows Security Center, open the Windows Firewall settings dialog.
   • (Windows Vista) Click Windows Firewall. Then click Change settings. You may receive a User Account Control pop-up. Click Continue.
2. Click the General tab.
3. Allow connections:
   • (Windows Vista) Deselect the checkbox for Block all incoming connections.
   • (Windows XP) Deselect the checkbox for Don't allow exceptions
     
     
     
4. Click the Exceptions tab.
5. If Mozilla Firefox is listed, select it and click the Delete button.
6. In the Delete a Program dialog, click Yes.
7. Set Windows to alert you when it blocks a program:
   • (Windows Vista) Select the checkbox for Notify me when Windows Firewall blocks a new program.
   • (Windows XP) Select the checbox for Display a notification when Windows Firewall blocks a program
     
     
     
8. Click OK.
9. Now, try browsing to a web site with Firefox. You may receive a Windows Security Alert dialog asking if you want to block Firefox. Click Unblock.

Based on information from Firewalls (mozillaZine KB)