then from the slide out
choose customize ..... the last option
and see if the back button is in the Left Box...
select it...click Add...it will be then in the right box...
the right box shows all the icons/options you put in your toolbar..
also clicking the RESET button , resets it to its default state.... which then includes both the back as well as forward arrow...
after transfering the back arrow from the left box to right box....click close
and you will find the left arrow in your toolbar.