Yes, the scroll wheel itself moves in a horizontal plane, but it triggers a vertical action on the computer screen. Nobody with any experience handling a mouse is thinking about the mouse itself, but only about the effect it will produce on the screen.

The "move towards" / "move away" analogy falls apart because the screen is a vertical plane. "Up" is more intuitive than "away", and "down" is more intuitive than "towards". You may even see an example of this intuition in how people talk about using the scroll wheel: nobody says "scroll towards" or "scroll away". The FF people who came up with the backwards convention must have been looking at a mouse without thinking about its common use cases.