Many modern mouses have two additional buttons on the side (thumb buttons). These buttons are very useful for navigation: usually applications (eg. Firefox) interpret these as Back and Forward actions.
This is supported by modern window managers (eg. Xorg) but not all applications that could benefit from this do actually use it (for instance Nautilus).

A workaround solution exists by using "translators" ie. small utilities that translate these button presses in key presses recognised by the application (ALT Left_arrow etc.). Beside the waste of system resources, inexpert users may find it difficult to find/install/configure such utilities.