How unfortunate! Super smooth scrolling in Firefox is among the arguments in favor of the Wayland transition. It is super smooth not only on modern AMD's but ancient Radeons.
It may not help, but try env GDK_BACKEND=wayland
Not much Firefox related, but a general observation: sometimes, when you hit into some very specific environment combo Wayland gets extremely choppy scrolling. Usually it also involves a fractional scaling, try integer display scale instead.