Do you have OpenH264 Video Codec provided by Cisco Systems, Inc. installed in about:plugins ?

To reset the mimetypes you can try to change the mimetype that cannot be found with these instructions:

• Applications panel - Set how Firefox handles different types of files

This means that the HTML5 media player is used, but your computer doesn't have support for the various media files that are available. This may be a missing codec or basic MP4 support.

You can open the Web Console (Firefox/Tools > Web Developer) for error messages.

• https://developer.mozilla.org/Tools/Web_Console

See also Windows Media Feature Pack for N and KN versions of all Windows 8 editions in case you use such a Windows version:

• http://support.microsoft.com/kb/2703761