In case you are unaware, or you were caught with your pants down, Firefox 2 has changed the preference to how the backspace key acts. By default in 1.5.0.* or earlier, the backspace key used to take you back/forward through pages. Now, it acts as PGUP/PGDN. This seem a little counter intuitive to me. Why would a key with the work 'back' in it, go up or down?
At any event, luckily, this can be changed fairly easily. Pull up about:config in your address bar, and change browser.backspace_action from '1' to '0'. Any other number will have no behavior when pressed.
Hope this helps.