I checked our support logs and found a case similar to yours: Here's what our support guru suggested:
"This must be related to firefox and what you have associated with different content types. In this case you may have an alert that is supposed to be playing a sound and quicktime is set up to be the application to play that sound. You’ll need to take a look at the active alerts you have when it wants to do this and check your browser settings."
Let me know if that works for you.