I was trying to watch the welcome videos today to set up our new account, and none of the videos were playing! I deleted my cookies, updated my extensions, and turned off all pop-up blockers for Time4Learning. Still nothing, yet the videos worked on Safari. After looking at several links through Google I finally found the answer, and thought I would share it in case anyone else has issues since I think this latest version of FF was recently released.

This article here explains that FF has now enabled "mixed content blocking." So if you are visiting T4L securely (instead of "http" your address bar has "https") then the YouTube videos might not play since FireFox considers them unsecure content on a secure page. Clicking the "shield" icon next to the address bar allows you to turn this feature off. The article shows what that looks like.

Hope this helps!