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