Comment 1 for bug 13889

It's a feature for people not using a mouse. If you press "alt-r" on your
keyboard then that option will be selected (or unselected). If you look
carefully in that menu, all 4 options in that menu have at least one letter with
a line under so that all 4 are selectable via the keyboard: Alt-m, alt-i,
alt-r and alt-a.