I have the latest versions of firefox installed on both my iMac and my MacBook Air. Both computers running the same IOS. Both Firefox versions have exactly the same preference settings. The iMac gets an occasional 502 Bad Gateway notification and will not go to that website. The MacBook Air does not get these issues. Both computers are running the same anti-virus software. This message started happening 3-4 weeks ago and is baffling as you cannot get to that webpage on the iMac but at the same time I get to it fine on the MacBook Air. What is causing this issue??