Firstly, click the "three bars" icon top right of the Firefox screen, then click "Add-ons", then "Plugins".
Have you got "Shockwave Flash" in that list? If so, is the drop-down box to the right of the screen showing "Always Activate"? If not, choose that option & try watching a video again.
If you *don't* have "Shockwave Flash" in the list, you need to add it, so use this link -
https://get.adobe.com/flashplayer/
*Untick* the 2 "Optional Offers" then click "Install Now".
Click on the blue arrow near the top right of the screen & you'll see the download - click on it to install.
You might have to restart Firefox when that's done - afterwards, try watching a video again & if it's still not working, come back to this thread & someone will be able to help further.