I have all the correct files uploaded (I think) on the website and the flash player loads under IE7. When I try to view the page in Firefox, the flash player gets stuck loading and never finishes. All I ever see is the circle of black bars spinning around.
I even tried embedding the player, but got the same result.
Here's the weird thing. All of the mp3 & media players on www.jeroenwijering.com work fine under both IE7 and Firefox.
Any ideas?

You probably did miss part of the embed code. If you use <embed> and <object> tags, make sure to add a 'flashvars' element to both of them!