Comment 2 for bug 1113490

apt-cache show midori|grep -i provide shows it provides www-browser, as does firefox and chromium-browser , so it feels like they should all be happy.

firefox does provide gnome-www-browser, but chromium-browser doesn't, so I can't see that's the problem

Do you have either of:
  djview-plugin
  gecko-mediaplayer
  mozplugger

installed?
They each seem to have an explicit list of browsers they work with in their depends?