D-Pad. (Sorry to those of you with an N810 - that's a big reason I won't get one)

And if a gesture is needed to show the controls, it should not be right-to-left: that would be too useful to just mean Back. (Forward is much less needed, so left-to-right can be considered free...)

Edit: I have not tried Fennec, I don't know what the current controls are or if they can be customized.