I came across these tweaks while searching for a solution on how to fix my issue with autoscrolling (which turned out to be a software issue, not Firefox). I’ll admit that I never really use the mousewheel (scroll wheel) in Firefox for any other purpose other than scrolling. The default behaviors when holding down certain keys and using the scroll wheel:

• Scroll Wheel Only – scrolls per wheel notch by six lines
• ALT+Scroll Wheel – scrolls per wheel notch by one line
• SHIFT+Scroll Wheel – navigates forward (up) or back (down) through the history
• CTRL+Scroll Wheel – increases (up) or decreases (down) the zoom

These behaviors can be changed for any version of Firefox via about:config. The preferences which control the behavior of the scroll wheel when used with certain keys (mousewheel.*.action) are:

• mousewheel.withnokey.action – scroll wheel only (not holding down ALT, CTRL or SHIFT keys)
• mousewheel.withaltkey.action – scroll wheel while holding down the ALT key.
• mousewheel.withshiftkey.action – scroll wheel while holding down the SHIFT key.
• mousewheel.withcontrolkey.action – scroll wheel while holding down the CTRL key

For each of the above preferences, there are four possible integer entries:

• 0 – scroll by lines (as set in mousewheel.*.numlinesof entry ^) of text in the browser window. Default behavior of the scroll wheel only (no keys used)
• 1 – scroll by pages (as set in mousewheel.*.numlinesof entry ^) of text in the browser window. Disabled by default
• 2 – move forward or backward in the browser history. Default behavior of SHIFT+Scroll Wheel
• 3 – zoom control. Default behavior of CTRL+Scroll Wheel

^ Important: If a change is made to any of the mousewheel.*.numlinesof entries, the corresponding mousewheel.*.sysnumlines entry must be set to FALSE. Failure to change the entry will result in Firefox ignoring the modified value in the corresponding mousewheel.*.numlinesof entry. Examples:

• Changing the value for mousewheel.nokey.numlinesof (scroll wheel only) from the default of 6 (lines per notch) to 10 (lines per notch) also requires the entry mousewheel.nokey.sysnumlines to be changed from TRUE to FALSE.
• Changing the value for mousewheel.alt.numlinesof (ALT+scroll wheel) from the default of 1 (lines per notch) to 2 (lines per notch) also requires the entry mousewheel.altkey.sysnumlines to be changed from TRUE to FALSE.

Source: ExtremeTech