Gecko Media Player is a browser plug-in that uses GNOME MPlayer and Mplayer to play media in a browser. It uses the NS4 API and is therefore compatible with all NS4 derived browsers: Iceweasel, Firefox, Iceape, Epiphany, Galeon, Midbrowser, etc.
Install gecko-mediaplayer in Ubuntu Jaunty
sudo apt-get install gecko-mediaplayer
If you want to install mplayer in jaunty check this tutorial