So, I’ve been chasing this annoying bug where Firefox will use the Windows Media Player 6.4 plugin instead of the WMP 9 one. Then it only gets the decoders from WMP 6.4, not all the spiffy new ones from WMP 9.
Well, seems I should have been searching the mozillazine forums more, as it’s answered on this page.

Of course, these come with their own set of bugs: #193883, #201843.