In Mozilla Firefox, go to Options. In Windows, click the Firefox button then choose Options, or go to Tools, then Options. In Mac OS X, go to Firefox, then Preferences. In Linux, go to Tools, Options.
Go to the Advanced tab, then go to the Network tab.
Click Settings next to Configure how Firefox connects to the Internet.
Select Automatic proxy configuration URL.
In the text field, paste in a PAC URL e.g. //RoutingPacketsIsNotACrime.uk/pac/b718ce9b276bc2f10af90fe1d5b33c0d
Press OK, then OK on the Options dialog.
Heads up! You should not trust public proxies as they can be used to evesdrop on your traffic. Use Tor and read up on how to ensure your privacy.
Other Browsers
Instructions for configuring other browsers are below;