Rated 5 out of 5 stars

Really great addon. There's only one thing I would like to see fixed:
I can't scroll horizontally with my mouse/touchpad. It keeps scrolling vertically. You can check the direction by comparing event.axis with event.VERTICAL_AXIS, see https://developer.mozilla.org/en/Gecko-Specific_DOM_Events#Determining_the_scroll_direction
Additionally, you could maybe add an option to invert scrolling direction when pressing shift.

This review is for a previous version of the add-on (0.20). 

>I can't scroll horizontally with my mouse/touchpad. It keeps scrolling vertically.
I have (hopefully) fixed horizontal scrolling in 0.21.
>Additionally, you could maybe add an option to invert scrolling direction when pressing shift.
Why would you need that?