Wrong Firefox Scrollbar Behavior (Ubuntu Mate 20.04/20.10)

In Firefox on Ubuntu Mate 20.04 and 20.10, when I click the left mouse button on the edges of the right side scroll bar with the intention of holding the bar, the effect is as if I were clicking above or below the bar, that is , it goes down as opposed to just being held. Fix this, please.

The right scroll bar has two colors, green and white. green seems to be the visible window of the page and white seems to be the full page. If you left click on the green, it allows you to scroll the page up/down. But, if you click on the white part, the page will scroll and the green bar will come closer to the mouse pointer. Just tested on FF, um20.04

The problem occurs when I click on the left or right edges of the green bar, not above or below.