That also happens with me, what ive found is you just have to go back to IE to use WMP. WMP plugin does'nt work with Firefox, you know why? Because WMP is created by..yep you guess it, Windows = Microsoft (like nearly everything else)
So why would MS let the Firefox company(whoever they are, i forget i think its mozilla) let them, they are against, they are battling, so to say, whos better and how many users can they get. Why would they combine, see its about the money and whos more popular. etc etc..
Which sucks in the 'run because the users(us geeks and all) miss out on the full benefit(s) of the browser(s)or just brower that is Firefox.
All because of greed and all that.
Well thats my view on it, i could be wrong, so if it is id like to know the reason. Veeery interesting!