HTTP/2 for Firefox is currently under development.
HTTP/2 draft support is included in routine builds of firefox, enabled by default in firefox 34 and later. To get the most up to date support you should use the nightly channel.
The current implementation is of draft-14 with compression-09 [august 6 nightly]
Firefox will only be implementing HTTP/2 over TLS - and so far that means for https:// schemed URLs (see below for http:// with TLS support). It does enforce the protocol's >= TLS 1.2 requirement - if a server negotiates HTTP/2 with a lower TLS version it is treated as a protocol error. (there is a preference for changing that for testing purposes if you need it.)
This build supports basic push and continuations and both alpn and npn.
feedback of any sort welcomed to email@example.com and firstname.lastname@example.org
There is a separate, more experimental, build available that supports HTTP/2 draft-12 for http:// URIs using Alternate-Services (-01) and the "h2-12" profile. Sometimes this is known as opportunistic encryption. This allows HTTP/2 over TLS for http:// URIs in some cases without verification of the SSL certificate. It also allows spdy/3 and spdy/3.1 for http:// URIs using the same mechanism.
That code is at https://hg.mozilla.org/try/rev/3d48401236e4 and https://github.com/mcmanus/gecko-dev/tree/oe10 with downloads at https://email@example.com