Just scroll horizontally on your touchpad or mouse wheel to go back and forward through your browsing history.
On Mac OS, Firefox supports back and forward via two-finger swipes on the trackpad. Scrolling Gestures enables similar gestures on any computer with a horizontal scrolling input, including touchpads and mice with tilting scroll wheels. As you complete the gesture, a feedback icon slides across the top of the page.
Scrolling Gestures is designed not to interfere with horizontal scrolling; the gesture only activates when there is no more scrolling in that direction.
Scrolling Gestures is not designed to be used on versions of Mac OS that support the "swipe to navigate".