How is that even possible? You can thank opportunistic encryption, a technique, which encrypts the communication when connecting to another system. As a result, Firefox will route HTTP (port 80) requests that are usually sent in the cleartext to a port of server administrator’s choice. In addition to that, users won’t experience any delays as connections will be fully established before they are even used.
The only requirement? A server must support HTTP/2 protocol and specify the AltSvc header.
As far as other important Firefox 37 changes go, it improves YouTube HTML5 playback on Windows as well as WebGL rendering performance, uses HTTPS for Bing search and improves protection against site impersonation via OneCRL centralized certificate revocation.