Well, I would guess updateProgressBar, but this would be a new method, because it does not exists in the default Container class and is also not overriden in Furnace (there it is created). But this method seems also to be just there, but not used... atleast in TileEntityFurnace and in the GUI and in the Container itself, eclipse couldn't find any place where it's called.
It does override updateProgressBar and it does exist in Container. ContainerFurnace just doesn't have the @Override over it.