Problem: Content Part Multimedia with a wmv other movie. Video is not playing on Firefox Windows. On IE or Firefox/Safari Mac, all is normal.
Solution: Don't provide a alt image for the Video. If you do AND FF is missing the player plug-in, FF will only show the alt image and doesn't prompt for the neccessary plug-in installation. You'll see this behavior only if the plug-in is missing. Once installed, it doesn't matter if there's a alt image or not.
Perhaps everybody knew this, and all the others hopefully will find this thread while searching for it