Just compiled version of Mobile Firefox for N9 with gstreamer support.
With small patch it actually works just great.
Package for N9 available here:
http://romaxa.info/fennec/14-gstreamer/fennec_14.0_armel.deb
Tested on:
http://camendesign.com/code/video_for_everybody/test.html - h264 video
http://jplayer.org, also tested with mp3 simple test page
http://romaxa.info/tests/mp3/test.html .
Still problem with codecs which gstreamer on N9 does not support, like WebM and OGG. Probably could be installed as extra plugins but I'm not sure.