Maybe this answer comes a little too late, but it might help someone who googles about this issue.
I've recently installed elementary OS that comes with Midori as default browser. I first tried installing Ubuntu restricted extras:
sudo apt-get install ubuntu-restricted-extras
It worked with SOME youtube videos, but not the ones embedded on facebook. Also, when I tested flash player plugin (here: http://www.adobe.com/software/flash/about/), it said "missing plugin".
Then, I added this line: *export MOZ_PLUGIN_PATH="/usr/lib/mozilla/plugins"* to configure Midori's flash path (this is for elementary OS. Other distro might use another path)
sudo nano /etc/profile.d/bash_completion.sh
It still didn't work, so after googling a little more, found this script: https://gist.github.com/lewisgoddard/6223985, wich solved all the issues :D
Note: you might need to restart your browser for changes to take effect.