This means you can get the source for Firefox and build it with support for <video> and <audio> by using the configure flag ‘–enable-media’. Currently the media support is disabled by default so it won’t appear in the nightly builds. At some point this will be changed and it will be enabled by default.
The patch that has landed does not yet include a backend decoder. It won’t play any videos as a result. That will be fixed when the various backends are landed:
In the meantime you can apply the patches from those bugs to the mozilla-central source to get a video build that decodes video.