The best list of ntl proxies seems to be 
the Forum one, akane1984.
To specify a proxy in Firefox, go to Tools -> Options -> General and click the Connection Settings button.  select Manual proxy configuration, enter the proxy you want to use in HTTP Proxy _only_ and enter 8080 as the port.  It's also a good idea to enter any IP addresses that you might want to access without a specified proxy (eg. those for your modem or router) in the No Proxy for box.
The click OK and OK: you're done.
If you want to be able to switch proxies on the fly, try the 
http://www.roundtwo.com/product/switchproxy extension.  You can enter several proxies and select which one you want to use with a couple of clicks.  Seems to me that Firefox was made for ntl!  ;-D