I'm browsing site via HTTPS Site supports HTTP2. This is clear by FirefoxDebugger Site logs (i am admin) confirms my browser is using HTTP2 When I browse site via GET/HEAD requests - absolutely no problems. Site uses default HTTP2 timeout == 180secs This works. I open netstat|grep 443 and see after 3minutes that TCP connection to my client browser is closed. When I make GET request after timeout and closing TCP connection - all ok, connection is reestablished and new 180sec timeout is created. BUT (!!!) When I make POST request after timeout and connection closed - I have an "ERROR making secure connection to server". I have checked Chromium browser - they are ok, GET/POST requests does not generate errors after timeout. This is definitely FireFox bug.