How to Fix NS_ERROR_NET_INADEQUATE_SECURITY and ERR_SPDY_INADEQUATE_TRANSPORT_SECURITY in IIS on Windows Server 2016

If you've recently upgraded an IIS web server to Windows Server 2016, you may come across a NS_ERROR_NET_INADEQUATE_SECURITY error in Firefox or an ERR_SPDY_INADEQUATE_TRANSPORT_SECURITY error in Google Chrome. There are a few reasons for this and the main one is that IIS in Windows Server 2016 turns on HTTP/2 by default and only falls back [...]