I just gave Firefox 30 beta7 a shot and now I'm absolutely sure what is needed:
For all Firefox versions you need gst-plugins-good
If you have Firefox <30
and gstreamer 0.10.x
you need: gst-ffmpeg for H264 videos (and of course, ffmpeg)
If you have Firefox >=30
and gstreamer 1.x
you need: gst-libav but you can also use gstreamer 0.10.x if you built that.
For Firefox <30 you have to pass --enable-gstreamer, for Firefox >=30 --enable-gstreamer=1.0 if you use the newer version. (At the moment the Firefox 30 Beta version doesn't run with GST 1.x, I've tried it)
After that, you can test it here: http://html5test.com/
Now that's what I call a little mess. But it's the right step for banning Flash from your system.
Building Firefox isn't that funny either, the build took almost all 4GB of my RAM.
Edit: Note, that you have to build gst-plugins-ugly
for using mp3 streams, for example on soundcloud without Flash