MPlayer is required to watch Windows media formats through your web browser.
I also use MPlayer exclusively to watch DVD's, technically the GUI for MPlayer is gmplayer.
The simplest way to install MPlayer is through the Packman software repositories:
I use both repos. I think MPlayer is actually in the Packman 10.2 repo.
Anyway, you will need to get MPlayer fully operational in order to watch Windows media formats inside Mozilla Firefox (and possibly the Konqueror web browser).
Every time you start gmplayer, you might see some bullshit error about a font (subfont.ttf). Just make a new text file in the folder that it shows you, rename the text file to the name of the font that it is looking for and then point to it in settings. It's annoying, but it's not a show stopper. eg: