HTTP/2 for Firefox is currently under development.
HTTP/2 draft support is included in routine builds of the nightly firefox-31, aurora firefox-30, and beta firefox-29 channels. It is disabled by default.
To get the nightly channel, download from https://nightly.mozilla.org/
To get the (slightly more stable) aurora channel, download from https://aurora.mozilla.org/
To get the (even more stable) beta channel, download from https://beta.mozilla.org/
To enable, go to about:config in the location bar and set network.http.spdy.enabled.http2draft and security.ssl.enable_alpn to true
The current implementation is of draft-10 with compression-06 on nightly, and draft-09 with compression-05 on aurora and beta.
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 1.1 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 build that implements draft-11 (h2-11) with compression-07 that has not yet landed on mozilla-central. It is lightly tested, has no unit tests yet. That code is at https://tbpl.mozilla.org/?tree=Try&rev=e09f5eaabea9 with downloads at https://email@example.com
There is a separate, more experimental, build available that supports HTTP/2-draft09 for http:// URIs using Alternate-Services (-01) and the "h2t" profile. Sometimes this is known as opportunstic encryption. This allows HTTP/2 over TLS for http:// URIs in some cases without verification of the SSL certificate. That code is at https://tbpl.mozilla.org/?tree=Try&rev=59983e67b49e with downloads at https://firstname.lastname@example.org/ .. if you need a server to try this with, you might consider https://github.com/molnarg/node-http2