When you try to load sites that don't exist or that you haven't opened recently with a proxy auto-config file (PAC),
Firefox may hang. To see if you're using an automated proxy configuration file, do the following:
Select Settings from the menu button.
Go to the Network Settings section of the General panel.
Go to the Settings tab.... The Connection Settings dialogue box is displayed.
You're utilising an automatic proxy configuration file if Automatic proxy configuration URL is enabled. This setting should not be disabled since it will prohibit you from accessing the Internet. Instead, give your network administrator this workaround.