I'd be willing to wager the X Factor is graphics card configuration and a card optimized for high performance 3D rendering (including having the drivers installed properly to leverage those features).

There's a bit of a correlation between Firefox users and users who are mostly content that their hardware supports explicitly a 2D desktop GUI and is not optimized for anything fancier. But nowadays, the browser itself is a 3D application because the 3D compositing pipeline is also the high performance 2D compositing pipeline.