I have problems with videos and firefox also. Some sites they wont play
Firefox plugins are blocking the script on those videos.
The browser addons I use Ad block and No Script will both stop the scripts from running on some sites. If I trust the site I click the no script icon and click allow all this page or temporarily allow all this page, that usually corrects the problem. Ad block is simular, you have to approve those sites like youtube otherwise the vids will not play.