In Firefox, when I start to move my mouse right, vertical scroll bar appears. Same is true for horizontal one when I move my mouse down. It happens even before

In Firefox, when I start to move my mouse right, vertical scroll bar appears. Same is true for horizontal one when I move my mouse down. It happens even before I get closer to any of the scroll bars. So, a wheel/trackpad is not required per se. Moreover, almost all mice I used except the most basic ones have horizontal scrolling capability.