Firefox scrollbar revert to original behaviour?

What you want is called legacy scrolling by the GNOME developers:
Edit or create ~/.config/gtk-3.0/settings.ini to have [Settings] as the first line and gtk-primary-button-warps-slider=false on a subsequent line by itself. This works in most gtk3 applications including Firefox but is said not to work in some. Alteatively, right-click instead of left-clicking works in all applications. And, if you want legacy scrolling system-wide, sudo edit /etc/gtk-3.0/settings.ini.

Whenever you edit settings.ini, the change maybe effective only after closing and re-opening Firefox. You may even need to log out and log back in.