A browser isn't really a program any more but a platform for running other programs. Like how javaw.exe is really Minecraft, firefox.exe is really YouTube. Go to about:processes to find more detail.
If I want to watch streaming videos, I don't have a choice about how I do that.
Fine Firefox is basically a bloated YouTube app. That doesn't change the fact that it is inefficient (from the pov of my CPU) for doing that.