That would be the software driver for your mouse to control how the wheel clicks and scrolls.

Mine clicks about 3/4 line on this site. To see whether yours has been customized:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful or accepting the risk.

(2) In the search box above the list, type or paste delta and pause while the list is filtered

(3) If the mousewheel.default.delta_multiplier_y preference is set to a value other than 100 you could double-click and edit it to 100 and see whether that works for you.

Testing in Firefox 62 on Windows 7, 200 doubles the distance, 300 triples the distance, etc., while 50 halves the distance, compared with 100.

Any difference on yours?

Edit: I don't know what that "multiplier" is multiplying. As WestEnd states, it probably is modifying how Firefox handles a value set externally to the browser.

I checked all of the delta entries in about:config and they're all set to 100%.

It sure would be nice if there was some documentation for all of these arcane values in about:config; the names of most of them provide little or no clue as to what they do, and the same goes for the values that they can accept. :-(

What if you try a reduced value such as 50 for 50%?

I haven't looked up most of these settings, sorry. This article was somewhat useful:

https://wiki.mozilla.org/Gecko:Mouse_Wheel_Scrolling

P.S. That suggestion was meant ONLY for the mousewheel.default.delta_multiplier_y preference ("y" indicating the vertical dimension).

Try to set mousewheel.default.delta_multiplier_y to 33 to change the scroll behavior from 3 lines to 1.

WestEnd said

That would be the software driver for your mouse to control how the wheel clicks and scrolls.

Thinking like a Windows user when this is on Linux ;)

jscher2000 said

Edit: I don't know what that "multiplier" is multiplying. As WestEnd states, it probably is modifying how Firefox handles a value set externally to the browser.

No just WestEnd thinking there is the usual software like Setpoint (no such for Linux) that does it.

There is the mousewheel Preferences. http://kb.mozillazine.org/About:config_entries#Mousewheel.