The VLC Media Player is an open source, cross-platform media client that supports the vast majority of media formats. Just check out the features page to see the list of them.
To install VLC, open a terminal window and use the following command:
sudo apt-get install vlc vlc-plugin-* mozilla-plugin-vlc
Once installed, you can find the VLC player in the Sound and Video menu under Applications.
The default skin is pretty simple, but it gets the job done:
You will need to restart Firefox to be able to use the plugin.