Open about:config and set
mousewheel.default.delta_multiplier_y to negative, e.g.
-100 if the current value is (the default)
This should only be necessary on platforms where Firefox isn't honouring the global setting. For instance on a Mac or Windows it's not necessary if you've set the scrolling direction to reverse globally, but on Linux, Firefox doesn't honour KDE's setting. I haven't tested GNOME ….
The default setting is fine for an actual mouse-wheel, but when using a touchpad to two-finger-scroll (or a trackball), it's more Natural for the page to scroll down when you swipe your fingers up: it seems more like how a paper page will shift.