Many mice nowadays have special buttons to navigate back and forward in browsers. Unfortunately, xchm 1.17 does not make use of them.
On linux, mouse button 8 goes backward and button 9 forward. (1,2,3=left, right, middle, 4+5 are vertical scrolling, 6+7 horizontal scrolling, 8+9 back/forward).
Pressing them on the browser widget should have the same effect as using the back/forward toolbar buttons.