i don't think there's a, ummm, windows media player plugin for firefox on linux... granted i'm not 100% sure about that, though... who knows what could be run in wine these days... either way, if you have (for example) mplayer or xine installed you can use windows codecs with them to playback most of the popular formats, like windows media 9, etc... i know there's an mplayer plugin for mozilla/firefox here:
http://mplayerplug-in.sourceforge.net/
and i think there's one for xine/gxine somewhere also...