Comment 3 for bug 365570

I have the same problem.

For all users of the computer, a proxy is indicated in the http_proxy environnement variable. They enter a pair of id/pwd, in a pop-up authentication window, then they want to view some web page with Firefox.
http_proxy=http://ip_proxy:port/

For apt-get, I put in the /etc/apt/apt.conf file a line as :
Acquire::http::Proxy "http://an_id:a_pwd@ip_proxy:port/";
(with rights 640, root as owner, admin as group ; also, other users can't see the content of this file).

Cause this bug, this config don't works.