This issue has plagued me ever since I switched for Firefox. I have NEVER encountered this issue with any other browser. Once every week or two I will encounter a page with an MP3/MP4 file and will be presented with this error. I have already tried clearing cache, disabling addons, reloading, etc, but I guarantee the "best answer" will just be some rehashing of these steps. Something is fundamentally broken with this browser, and there are hundreds of people across various sites reporting it. Fix it.