HTML5 Videos- won't play on Firefox 21.0

I’m sorry this is so late. This plugin is no longer in active development. If you’re still having this problem (and using this plugin), it sounds like there’s a problem with the OGV file, which Firefox uses. You might try to remove that source all together, as Firefox should also support .webm. Make sure your server can serve the webm mime type as well. Instructions for enabling that can be found here: https://www.paulund.co.uk/change-wordpress-upload-mime-types.