Why do you want to use 10% increments as you may not see any change as those changes are too small ? Note that zooming by rotating the mouse wheel with Ctrl pressed adjusts the zoom in 10% changes starting with the current zoom value. You can set your own zoom values via this pref with a 5% resolution (x.y0 and x.y5). If you want to use 10% then use one decimal (x.y), but it is not a good idea to specify them all (i.e. skip some). *about:config => toolkit.zoomManager.zoomValues Otherwise you can only set a maximum and minimum zoom value. *zoom.minPercent = 30 *zoom.maxPercent = 300