There are two methods you can use to get this working:
- Autoscrolling (middleclick scrolling)
- IMwheel to emulate horizontal scrolling
Autoscrolling
It sounds like autoscrolling is disabled in your installation. You can re-enable it by going into Edit->Preferences->Advanced and checking "Use autoscrolling".
IMwheel
This is a method I was just trying out myself to get Shift scroll working.
First, install imwheel:
$ sudo apt-get install imwheel
Then copy-paste this into your $HOME/.imwheelrc:
"Firefox" Shift_L, Up, Button6 Shift_L, Down, Button7 Shift_R, Up, Button6 Shift_R, Down, Button7
Then start imwheel:
$ imwheel
It seems to work for me, but if I scroll too quickly, some of the events get through imwheel and it starts scrolling diagonally instead.