In Unix and Linux, recent versions of Firefox had a change in scrollbar behavior. In older versions, clicking in the scrollbar trough would move the scrollbar thumb one windowful toward the spot you clicked, scrolling the window just one windowful in that direction. Now, the default is that clicking in the scrollbar trough makes the thumb jump to the point you clicked instead, often causing the window to scroll dozens of windowfuls instead on long pages.
Believe it or not, the cause isn't actually Firefox itself. It's GTK3.
In GTK3—unlike previous versions of GTK, other X11 toolkits, and other operating systems—left-clicking the scrollbar trough makes the thumb jump to that spot, while right-clicking the scrollbar trough makes the thumb scroll a windowful toward that spot.
~/.config/gtk-3.0/settings.iniwith a text editor. If the file doesn't exist, create it.
If it has a
[Settings]section, add this line just below it:
gtk-primary-button-warps-slider = false
Otherwise, add your own at the end of the file, like this:
[Settings] gtk-primary-button-warps-slider = false
Quit and restart Firefox.