This is an issue with Firefox 26 and Flash video. Firefox is decreasing support for plugins such as Flash in each subsequent update.
First, please ensure your theme is up to date. Video-specific styling issues were addressed in earlier versions of this theme.
If issues persist, you may be having issues with browser rendering of Flash Video.
If you run a large site with visitor support, offer your visitors the same advice as below to ensure they have the best view possible:
- Go to http://www.youtube.com/html5 and click “Request the HTML5 Player” button to enable the HTML5 player.
- Reload the post and the video should now appear in the HTML5 player.
There are some things one can do to ensure Flash has the best possibility of displaying if HTML5 is still not solving it, ranging from updating Flash to disabling protected mode. See the following Firefox support post for more help: https://support.mozilla.org/en-US/questions/980466