Firefox has its own settings for Proxy configuration, which are separate from those used by IE.
I'm not familiar with NTL Medic, but you may be able to look at the Proxy settings in your Internet Options control panel and duplicate those in Firefox's settings:
Open the Internet Options control panel and go to Connections>Lan Settings. If you see Proxy configuration information there, try to replicate that information in Firefox's Tools>Options>General>Connection Settings window.