Microsoft has released a Windows Media Player plug-in so the player works with Firefox 2.0 on Vista.
Nick White posted on the Windows Vista blog:
The Windows Media Player team put a lot of work into evolving media playback on Windows Vista through the new Media Foundation pipeline, and has also been actively monitoring feedback on WMP and playback in general. While commentary has been mainly positive for Web playback through IE, we've noticed that there's still work to be done to make Firefox users able to enjoy their media content on Windows.
We couldn't respond as quickly as we would have liked to (we had to get Windows Vista out the door!), but now that it's shipped, the team has moved its attention to getting Firefox users up and running.
The new plug-in for Firefox can be found at Port 25, which is Microsoft's communications hub with the open source world.