Comment 7 for bug 360222

@Eugenia:

Until it gets fixed, you can use gecko-mediaplayer instead:

    sudo apt-get remove mozilla-mplayer
    sudo apt-get install gecko-mediaplayer

They're both in multiverse, so if you have mozilla-mplayer installed, you should be able to install gecko-mediaplayer:

See here for more info:

http://www.ubuntugeek.com/how-to-enable-the-universe-and-multiverse-repositories-in-ubuntu-804-hardy.html
https://bugs.launchpad.net/ubuntu/+source/mplayerplug-in/+bug/360222/comments/4