I have a VPN for work, some internal sites are inaccessible without this VPN. When I try to access them with the VPN turned off I legitimately get a 403. However, when I turn on the VPN firefox keeps showing 403 while other browsers work fine. This also works the other way around. When I turn off the VPN and try to go to one of the internal sites I don't get a 403 (as I should), instead firefox gets stuck at the page loading stage and nothing happens. There seems to be some kind of caching or optimization going on, but it's terribly interfering. Deleting cookies, caches, etc doesn't help at all. The only thing that helps in this situation is restarting firefox.