Comment 3 for bug 127514

Setting Option "ButtonMapping" "1 2 3 6 7" does map the 4th physical button to event 6 and the 5th physical button to event 7.

The problem is that the above mapping only works in firefox, and not in e.g. Nautilus and other applications with a back and forward button.
Firefox assumes that event 6 = back and event 7 = forward.
By default xorg assumes that event 6 and 7 take care of vertical scroll (Default: "ButtonMapping" "1 2 3 8 9 10 ...").

So the problem is that each application can decide for itself what events 6 and higher mean. Only event 1 to 5 are defacto standard.

I believe applications should always use event 6 and 7 for vertical scroll. All the additional buttons (event 8 and higher) should be left to the user to define, since they can do whatever you want them to do by e.g. using imwheel which maps a mouse event to a keyboard key combination depending on the application.
In that case a good default imwheel configuration that maps event 8 to the back button and event 9 to the forward button for frequently used applications should be enough. Then simply installing the imwheel package should make the back / forward buttons work and users are able to reconfigure them is they want these buttons to do something else.
Therefore we should be able to switch off firefox's handling of event 6 and 7 and leave this to imwheel instead. Does anybody know if this is possible in firefox?

Marking xorg task as invalid, nothing needs to change here.
Adding firefox and imwheel task.