However upon finishing my default installation, the back button is missing. I thought it's default when you don't have any previous link. But indeed it's missing. I tried to search for google, but the result was mostly related to RC version bug. They shouldn't have this problem on final release of Firefox 3!
After a while, I finally figured out it must be the layout issue. To solve this problem, go to view > toolbars > Customize...
You can either drag the button to the layout, or just click 'restore default set'.
Voila! Solved.