From MozillaZine Knowledge Base

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 an "Server not found" error when you try to browse to web sites.

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 {PATH()}Control Panel{PATH}.
    • __(Windows XP)__ Open the Windows Start Menu, and select {PATH()}Control Panel{PATH}. Your Start Menu may have the {PATH()}Control Panel{PATH} as an item under {PATH()}Settings{PATH}.
  1. In the Control Panel, open the Windows Security Center.
    • __(Windows Vista)__ Under the {DIV(class=button,type=>span)}Security{DIV} header, click {DIV(class=button,type=>span)}Check this computer's security status{DIV}.
  1. Open the {DIV(class=button,type=>span)}Firewall{DIV} header. Try to locate the following text: %%% __Windows Firewall is actively protecting your computer__
  2. If it is displayed, 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 {DIV(class=button,type=>span)}Windows Firewall{DIV}. Then click {DIV(class=button,type=>span)}Change settings{DIV}. You may receive a User Account Control pop-up. Click {DIV(class=button,type=>span)}Continue{DIV}.
  1. Click the {PATH()}General{PATH} tab.
  2. Deselect the checkbox for __Block all incoming connections__.
  3. Click the {PATH()}Exceptions{PATH} tab.
  4. If __Mozilla Firefox__ is listed, select it and click the {DIV(class=button,type=>span)}Delete{DIV} button.
  5. In the Delete a Program dialog, click {DIV(class=button,type=>span)}Yes{DIV}.
  6. On the {PATH()}Exceptions{PATH} tab, select the checkbox for __Notify me when Windows Firewall blocks a new program__.
  7. Click {DIV(class=button,type=>span)}OK{DIV}.
  8. 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 {DIV(class=button,type=>span)}Unblock{DIV}.