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!
An error occurred while saving the commentedmond 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.