If you use as minimalistic version of Firefox as I do (which means that the Navigation Toolbar is hidden and all the buttons are moved from it to the space next to the address bar), after installing Firefox 3 you will notice that not only are the Back and Forward buttons missing but also you cannot add them from the Customize Toolbar window.
This is because the buttons are already added to the interface but they are not visible because the Navigation Toolbar is hidden.
Of course, the a Back/Forward button (two separate buttons from previous versions have been replaced with a single one) can be moved from the Navigation Toolbar to the place of your choice.
I encountered this problem after upgrading Ubuntu 7.10 (Gutsy Gibbon) to Ubuntu 8.04 (Hardy Heron), but it has nothing to do with Ubuntu. It is caused only by the fact that old Back and Forward buttons have been removed and a new Back/Forward one has been added. And it is placed in its default location (which is the Navigation Toolbar).