It's often a dodgy extension which causes video problems. To see if that applies to you, click on the hamburger (three lines, top right). Click on Help (question mark, at the foot of the menu). Click on Restart with Add-ons Disabled. Then see if the videos display normally.
If that fixes the problem, enable your extensions one-by-one, to see which one is causing the problem. (via Hamburger > Add-ons).
If it doesn't fix the problem, note down the names of any extensions you want to keep (because you're about to remove them) and reset Firefox to its default status. Don't worry, you won't lose your bookmarks! (Hamburger > Question Mark > Troubleshooting Information > Refresh Firefox) . That should hopefully fix the problem. (You can then reinstall any extensions you need).