If you’re like me and think it’s really stupid that a mouse has a mapped “back” key instead of an extra button, you can easily remap it with x-mouse.
It’s freeware, it’s extremely light weight and efficient, cleanly starts on boot with no obtrusive ads or notifications, and has a whole suite of options including active-window-specific remapping.
I use it almost exclusively to remap the back and forward buttons on my trackball.