I'm trying to run a local IPython Notebook [1] in Firefox with a proxy-auto-config (PAC). It opens under http://localhost:8888/ I get an error message just as stated in [2] about a websocket connection failing (also when changing to http://127.0.0.1:8888/). Changing the advanced network properties to manual proxy settings with "no proxy for localhost, 127.0.0.1", the error disappears. I think that means Firefox tries to use a proxy for localhost when the PAC file is the active setting. I looked into the file and found function FindProxyForURL(url, host) { if (shExpMatch(url, "http*://localhost/*")) return "DIRECT"; if (shExpMatch(url, "http*://localhost:*/*")) return "DIRECT"; if (shExpMatch(url, "http*://127.*.*.*/*")) return "DIRECT"; } So, if I understand this correctly, Firefox shouldn't be using a proxy for localhost, but I get a different behavior than if I use the manual setting ignoring localhost and 127.0.0.1 -- why is that? There is no other mention of localhost in the PAC file. The user of stackoverflow in [2] has noticed the same problem. I also tried in safe mode, no difference. [1] http://ipython.org/notebook.html [2] http://stackoverflow.com/questions/19245200/ipython-notebook-websocket-connection-failed Using Firefox ESR 31.5.3 with Windows 7