GNOME Bugzilla – Bug 677084
No way to enter proxy credentials
Last modified: 2012-05-30 17:00:54 UTC
I'm not sure if it's significant but I'm not running NetworkManager. There's UI in "Network" within "System Settings" to specify the HTTP proxy, port, etc but nowhere to enter the proxy username and password. Obviously that can't come via a proxy autoconfiguration URL.
Did you try username:password@proxyname:portnumber ?
*** This bug has been marked as a duplicate of bug 646354 ***