Do you get Quicktime errors when you try to play an embedded multimedia file when viewing a website with Firefox? Have you already tried downloading and installing the latest version of Quicktime?
Well then, most likely the answer to your problem is that you have to enable the Quicktime audio and video plugins.
To test to see which types of files are not playing in Firefox for you, go here:
Once you know which files are not playing, then go here to see how to enable them: