In order to enable this feature,you have to navigate the hidden firefox settings..!!
In address bar type:
about:config
You will see a textbox and a long list of options and settings. In the "filter" textbox type:
mousewheel.withnokey.sysnumlines
Change the default value to "false". Then type:
mousewheel.withnokey.numlines
The default value is "1". Increase this number to have faster scroll speed in firefox. I prefer "6".
You don't even need to restart firefox to see the changes. They take effect on immediatly.