We have checked this through FireFox and the videos play as intended. Please go here and double check that your Flash player is enabled: https://helpx.adobe.com/flash-player.html
Also if possible can you please provide some of the videos you have opened and do not play so we can investigate this further? Thank you!edmond commented
I also use Firefox and I did make sure that the most recent version of Flash-player is installed. Yet, I continue to have trouble playing videos. The first video will play. But when I attempt to play the second video it will not work. I hear the sound, but the picture is frozen. If I log out and come back, again the first video will play, but not the second.