Some websites that use Windows Media does not work when using Totem as a player, but people have told me that replacing Totem with VLC in Firefox can sometimes make things work. So the question becomes: how do I replace Totem with VLC as a video handler in Firefox?
I don't recommend doing that. While VLC is one of the best players, the plugin is really bad. The recommendation on the other reply to change the Applications settings won't work. When you have multiple plugins installed, Firefox decides which one to use by the mime-type of the content. The only way I know how to force Firefox to use another plugin is to change the video object type in the page, but that is very complicated and not feasible.
I would recommend removing VLC plugin and Totem plugin, then installing gecko-mediaplayer.