Adjusting scrolling speed isn’t an ability we’re able to do in Gnome just yet. Hard wired into GTK is a set scrolling speed and it can’t be tampered. Thankfully though Firefox(and it’s derivatives) have created support for this.
In the location bar type
about:config and then type
mousewheel in the filter liine. Change
mousewheel.withnokey.sysnumlines to false by doubleclicking and change
mousewheel.withnokey.numlines to a number of lines to scroll with each click (I like about eight). I find that turning on smooth scrolling additionally helps to be able to keep focus.