I discovered when I upgraded to Firefox 18.104.22.168 that I couldn’t get the player to install properly from Rhapsody.com.
I’m certain the problem is on the Rhapsody.com end, and I’m alarmed they haven’t fixed it yet.
In the mean time, I’ve found a direct URL that you can use (at your own risk) to install the player extension in Firefox.
I used it and it worked fine for me.
You can download the Rhapsody Player Plugin here.