Firefox Nightly currently supports HTTP/2 protocol which is major revision of web’s protocol, this feature is disabled by default, you can by turning on two preferences in about:config.  “Firefox will only be implementing HTTP/2 over TLS – and right now that means for https:// schemed URLs. It does enforce the protocol’s >= TLS 1.1 requirement – if a server negotiates HTTP/2 with a lower TLS version it is treated as a protocol error.”

Enable HTTP/2 in Firefox

1. Visit about:config from location bar and press enter

2. Set network.http.spdy.enabled.http2draft and security.ssl.enable_alpn preferences values to true from false.

Separate experimental build supporting with HTTP/2 is available, you can download it from here.