Comment 111 for bug 1051559

Why it's not enabled yet in Ubuntu (https://bazaar.launchpad.net/~mozillateam/firefox/firefox-aurora.head/revision/1259#debian/changelog) :
"* Build with --disable-gstreamer for now, as this adds a hard dependency on
  the *old* gstreamer stack. Reenable this once Firefox has been dropped
  from the default install"