Podle návodu to nefunguje? Default je systémové nastavení v ovládacím panelu myši.

Type about:config into the location bar and press enter
Accept the warning message that appears and a list of preferences will appear
In the filter box type numlines
Double-click on the preference mousewheel.withnokey.sysnumlines to change its value to false
Double-click on the preference mousewheel.withnokey.numlines and change it to the number of lines you want to scroll