Escomatt,
I had the same problem. I solved it by reinstalling WMP10 over itself. That, in fact, is the recommended way to solve most problems with WMP (as per the wmp mini-faq).
If you don't have the full installation setup file just go to the Windows Media Player website and download it from there.
Before running the install it's a good idea to check a section of your registry. Go to Start, Run and type in regedit and open it. Look in Hkey, Local Machine, Software, Microsoft, Media Player, Shim Inclusion List. See if it has MOZILLA.EXE, PHOENIX.EXE, MOZILLAFIREBIRD.EXE, and FIREFOX.EXE listed there. If they aren't, add them.
Also check the Shim Exclusion List in the same area and make sure those files aren't listed. If they are, remove them.
Then just run the WMP10 setup.
There is a reg file that has been circulating called wmp9.reg. That file adds the references to the registry for you. But, it doesn't remove the references that sometimes show up in the Shim Exclusion List. If you don't want to add the files yourself, you might check the Bugzilla website for Bug #193883. I think there is a link in there for the wmp9.reg file. You can also read about my struggles with the problem near the bottom. I had the proper references, Firefox detected and had activated the proper plugins (about:plugins revealed that), but still got WMP6.4's embedded interface in Firefox web pages. I needed to reinstall WMP10 to get back the WMP10 embedded player.
Don't be concerned with the posts and links regarding the ActiveX Control for Firefox 1.0. You don't need, and shouldn't use that if you value the security the non-ActiveX Firefox provides over the ActiveX infected (!) Internet Explorer.
I think this problem develops when certain websites insist on calling WMP6.4 and once it does that even one time, then 6.4 takes embeds over. I've had it switch back to 6.4 several times. Reinstalling WMP10 puts back the association to the WMP10 embedded player. No fooling around with Options or File Type Associations helped me. I had wmp6.4 not associated with any file types and WMP10 associated almost all and still got the 6.4.
Good Luck. This was driving me crazy for a while!