I am using Firefox behind a corporate proxy which uses a fairly complex PAC file to route traffic. This is configured at the OS level and I typically have the "use system proxy settings" option checked. After a using Firefox for some period of time (generally between 1/2 and hour and 5 hours) external pages stop loading. All the internal pages that that don't need the proxy still load but external pages hang when loading. They appear to start loading the page but always hang before finishing. Once this has started this will happen to all external pages. Restarting Firefox fixes the issue. Going into settings and changing the proxy setting will fix the issue (e.g. I have the proxy url used by the system set up in "automatic proxy configuration URL" and swapping between that and "use system proxy settings" will fix the issue. I have tried downloading the PAC file locally and running off that as well, but I get the same issue (works for a time before I have to swap proxy settings). I'm slowly trying to add more and more sites to "No Proxy for" list in case it's some kind of circular lock without effect so far. It affects both Windows and Mac, and only Firefox (chrome, IE, Edge and Safari all work without issue). I'm not sure how to debug the issue further. I'd like to do some kind of trace route through Firefox to see if when the block hits it is always failing at the same point but I'm not sure how to do that. Where should I start?