How do I transfer music in Windows Media player from one computer to another?

-I tried using a flash drive, and was able to transfer the content over from one computer to the other. However, I'm having trouble with licensing. Microsoft gives me the option to retrieve license's. But when I click the download button I'm prompted, "Your computers netscape is not equipped with Window Media Service Plug in's." I have Mozilla Firefox as my web browser. Is there another method that I should try?