When you say "watching a video" I'm casually assuming it was a flash movie with a video in it, because that is by far the most common way to watch online videos now. What makes this possible (because flash is not built into Firefox or any other browser) is the flash plugin, from Adobe. And it's a piece of junk. It used to work relatively well in the past, before flash movies could embed videos. Adding video functionality to flash made it a lot more complex, and in result a lot more buggy.