*Note* This assumes you've gone into the Touchpad settings under Easy Mode (Settings → Touchpad → Scrolling) and enabled horizontal scrolling, and that you've done the Mozilla scrolling fix.
In my personal experience, I've found the vertical edge scrolling works fine when you turn it on but not its horizontal counterpart. From the get-go I found I literally had to dig the sides of my finger into the lower edge to get any sort of a response from the touchpad.
The trick to resolve this is in the xorg.conf file, under the synaptics settings for the touchpad's X-Y edge coordinates.
**NB - Back up your xorg.conf file first if you're inclined to fat-finger your keyboard: sudo cp /etc/X11/xorg.conf /etc/X11/xorg.conf.backup
*The following applies to both easy and advanced mode*
Access the xorg file in the editor: sudo kwrite /etc/X11/xorg.conf
Default should look like this (only first few lines displayed):
Section "InputDevice" Identifier "synaptics" Driver "synaptics" Option "Device" "/dev/psaux" Option "Protocol" "auto-dev" Option "LeftEdge" "1000" Option "RightEdge" "5400" Option "TopEdge" "1000" Option "BottomEdge" "4900" EndSection
For those who want to prove this to themselves, using the command 'synclient -m 1' in the terminal and placing your finger on the edges of the touchpad, you'll see that the coordinate of the edge on the right is actually somewhere around 6100-6200 (X), whereas the bottom edge is around 5300 (Y).
If we compare the actual value for the right edge vs. its value in the config file, there's a difference of approx 800 units (6200-5400). This means the default value of 4900 for the BottomEdge is set far too high - there's a difference of only 400 units between actual value and config value.
What we to do is make the synaptics file 'think' the lower edge is closer to the centre of the pad by lowering the BottomEdge number.
Change the BottomEdge value to somewhere between 4300-4400 (I've found this to be the best range) so the line reads something like this:
Option "BottomEdge" "4320"
Restart X windows with a quick ctrl-alt-backspace, and voila, responsive horizontal scrolling!