proxy works on windows but not on ubuntu

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http://askubuntu.com – i work on ubuntu 12.04 with a internet connection using proxy. Lately the proxy address changed, and now suddenly I cannot connect to internet whatsoever. Strangely, the same proxy adress does work on a windows machine. I know there is a bug concerning the system wide application of proxy settings, but that is not my problem. If i set the new proxy address in firefox it just does not connect. But in the network manager I can see that there is a connection. Any clues? Thanks, Rick Jansen, Amsterdam (HowTos)