"No video with supported format and MIME type found" error message means that Firefox wants to use the HTML5 media player to play the file and that the server doesn't send one of the possible with a supported MIME (content) type.

• https://developer.mozilla.org/en/Media_formats_supported_by_the_audio_and_video_elements
• https://developer.mozilla.org/en/Configuring_servers_for_Ogg_media

Thanks for responding. I'm sorry, I don't understand. The creator intended them to be playable with the HTML5 player, and others have been able to play them. Also, there is no server. The files are on my local machine.

Is the m4v file a playlist file that has the addresses (URLs) of other files that need to be played?

Are all those files also local files and do they at least have a supported file extension?

There's no playlist, just individual videos. They are all local (although there is an online version that gives me the same problem.

Firefox can't play files with the .m4v file extension via the audio or video tag as those are not one of the supported file extensions. You can only play such files via the object tag to make Firefox use a plugin that supports this file extension. You can force to use a specific plugin by specifying a type (type="application/x-mplayer2").

<object data="file://<path to file>" type="<to force a specific plugin>"></object>

Okay, thanks.