Firefox 3.6 on Ubuntu and I believe the back button's default was small (I.e., the same size as the forward button).
I know there are many fixes to make it small, however, I would like to make it big like the usual Firefox.
I would like to have the first image.
EDIT: the "Use Small Icons" box under Toolbar > Customize is toggled off. Never played with it before. The Back button was never large.