User Agent: Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 6.1; Trident/4.0; SLCC2; .NET CLR 2.0.50727; .NET CLR 3.5.30729; .NET CLR 3.0.30729; Media Center PC 6.0; Tablet PC 2.0; .NET4.0C; .NET4.0E; InfoPath.3; MS-RTC LM 8) Steps to reproduce: Firefox 24.3.0 ESR and previous versions, also tested several times with trunk versions. Windows 7 (and before XP), Firefox set to "Use system proxy settings". Windows itself is configured to use autoproxy. Start using Firefox in environment where proxy is required (corporate LAN) it works fine. Now suspend system, move to different place (home), resume. The problem has been present in all previous versions and is pretty much annoying, as it requires to restart Firefox thus losing any current activity. Actual results: Firefox never notices that proxy is no more valid. Internet Explorer on the same system works fine after small initial delay. I have waited for about half an hour without change - network trace confirms that Firefox still attempts to contact old proxy. Expected results: Firefox should notice that proxy configuration changed and stop attempt to contact proxy.

bug 939318 ought to fix this

I think this bug is back. Till some days ago "Use system proxy settings" worked fine. IE had automatic search settings, get the proxy.pac and firefox took these settings too. I could rebuild this problem on Windows 7 with the Firefox 40.2. When i changed the settings in IE to configuration skript (with pah to proxy.pac) the "Use system proxy settings" in firefox worked too. I think the problem is the automatic proxy settings search.

bagder's code should fix this