Hello everyone! I have a locally hosted Windows service which receives POST requests from a browser which has my web site loaded. Since this is cross domain communication I use CORS. This works perfectly fine for Windows 7, 8, and 8.1 for FireFox, IE and Chrome, with and without HTTPS. On Windows 10 this also works fine for IE and Chrome with and without HTTPS and FireFox WITHOUT HTTPS. The only scenario where I've problems is Windows 10 + FireFox + HTTPS. I tested with version 42, 43 and 44. HTTPS works in all cases with a self signed certificate, the corresponding root certificate is correctly installed in FireFox and I don't get any certificate warnings. My web site calls two different methods on the service and therefore has to send two OPTIONS requests. The first one passes quickly, but the second one takes up to 59s. Afterwards the following POST request is executed normally. I did some tracing in the service and it looks like that FireFox waits 59s to send that request. Once I incorporate Fiddler with the correct root certificate installed it works fine. Does anybody have an idea what might be going on? Could this be a bug in FireFox only occurring on Windows 10? Why does it suddenly work if Fiddler is running? Thank you for your ideas! Best regards, Benjamin