Open a new tab in Firefox, and enter into the address bar: about:config.

There may be a warning about the sensitivity of the advanced settings within, accept the risk and proceed.

There will be a search bar at the top, search for:

mousewheel.default.delta_multiplier_y

The default value seems to be 100. Change it to something like 30, and save with the checkmark on the right.

The updated scroll speed should be experienced immediately.

Mind you, this setting seems to influence both the mouse's scrollwheel and the two-finger scrolling on the touchpad. I don't know how could one configure the two independently from each other.