I followed the instructions in the solution and changed to appropriate plug in to true in the add ons menu. It did not repair the issue. The 'solution' also referenced manually moving the files to Firefox 21 but did not specify what folder to place them. Internet Explorer lets me play Crescendo. I hate IE. There is a plug in folder in IE that apparently has the appropriate one to enable play. Should the contents of the IE folder be copied and placed in the Mozilla folder somewhere???? Would love to have an answer. I can play all the other games on King but that one was a favorite. Thank you in advance for your time and consideration.