To find out which number corresponds to which button on your 7 button mouse, open a terminal console and type xev
A white box will appear. Put your mouse cursor into that box. Try to hold the mouse as still as possible, as any movement generates 'events' in the terminal window. Click a mouse button, looking at the output generated in the terminal window. You should see something like 'button 5' down in the text. Write down those associations as you discover them, so you won't forget or have to redo the proceedure.
Below is my custom setup for my mouse in my /etc/X11/xorg.conf file. I use a MS Laser Mouse 6000 USB. On my mouse, the numbers 4 and 5 relate to the scroll wheel rotation buttons and the 8 and 9 relate to the buttons on the left and right side of the mouse (not left click, right click). You want to associate those functions with the "ZAxisMapping" opton, using the number from your
This will give you normal scrolling functions on the scroll wheel plus 'back' and 'forward' functionality on those left side and right side mouse buttons while in your browser. Use the below example to setup your mouse in xorg.conf, substituting your numbers, and either remove or modify the Resolution option to suit your mouse's capability.
# generated from default
Option "Protocol" "auto"
Option "Device" "/dev/input/mice"
Option "Emulate3Buttons" "no"
Option "Resolution" "1000"
Option "Buttons" "7"
Option "ZAxisMapping" "4 5 8 9"
Option "ButtonMapping" "1 2 3 6 7 10 11"
edit: In order to make the changes to xorg.conf effective, you must restart the X server. Log out then back in or CTRL-ALT-Backspace to force the same.