A while back, I lost the ability to play Windows Media Player (WMP) videos with my Firefox browser. The WMP controls wouldn’t show up, and there was just a blank space where the media was supposed to be playing. I downloaded and reinstalled the latest WMP version, to no avail. Eventually I just bit the bullet and opened WMP content in Internet Explorer.
Thanks to MozillaZine, however, I have identified and fixed the problem. Turns out there is a conflict between the VLC plugin and the WMP plugin. The solution is to remove the VLC plugin (accomplished by deleting the npvlc.dll file from the Firefox ‘plugins’ directory). I exited Firefox, deleted the file, restarted Firefox, and voila! I could watch Windows Media files.
If you have Firefox and VLC installed, there is a test page where you can check to see if you have this problem.